From time to time I run a python loop through all the JJ2 levels or tilesets or something, to uncover some bit of useless information that people might still find interesting. Here's a repository of those. Let me know if you have any specific data-driven questions that could be answered this way.

The official JJ2 changelog for version 1.20a mentions "New level masks implemented. Reduces the risk of players getting stuck in the scenery." Here's a full list of every tile mask that was changed between 1.20 and 1.24x (left is original, right is patched):

Here's how many official levels each food pickup appears in:

Eggplant: 0
Cherry: 1
Thing: 1
Lettuce: 1
Cucumb: 1
Taco: 1
Cupcake: 2
Weenie: 2
Ham: 2
Banana: 3
Strawberry: 3
Soft Drink: 3
Icecream: 3
Fries: 3
Pretzel: 4
Grapes: 4
Milk: 4
Pie: 4
Pear: 5
Donut: 5
Lime: 6
Watermelon: 6
Soda Pop: 6
Chips: 6
Burger: 6
Pizza: 6
Chicken Leg: 6
Sandwich: 6
Cheese: 6
Candy: 7
Chocbar: 7
Apple: 8
Cake: 9
Orange: 10
Peach: 10
Lemon: 15

Eggplant does appear in CC99, though, in the user level XB-Kings-ea.j2l.

To compare, Here's how many levels uploaded to J2O prior to September 2017 each food pickup appears in:

Pretzel: 185
Taco: 196
Weenie: 211
Thing: 212
Pie: 219
Sandwich: 223
Lettuce: 234
Strawberry: 252
Soft Drink: 259
Cucumb: 277
Pear: 288
Eggplant: 289
Fries: 292
Milk: 295
Cupcake: 298
Soda Pop: 300
Watermelon: 301
Ham: 313
Lime: 318
Icecream: 332
Grapes: 353
Pizza: 362
Peach: 364
Orange: 403
Chips: 418
Chocbar: 426
Donut: 430
Banana: 444
Cake: 449
Candy: 474
Lemon: 483
Burger: 515
Cherry: 564
Cheese: 625
Chicken Leg: 695
Apple: 759

Rough ratios of custom tilesets to official tilesets according to my 1.23 cache (2009 and earlier), J2O (2010 to 2017), and TSF cache (2018 and beyond):

2001: 1.2545454545454546
2002: 2.1343283582089554
2003: 1.9014084507042253
2004: 1.320754716981132
2005: 2.4563758389261743
2006: 2.011904761904762
2007: 1.8
2008: 1.2894736842105263
2009: 2.6296296296296298
2010: 2.155844155844156
2011: 2.8461538461538463
2012: 1.901639344262295
2013: 1.8306451612903225
2014: 2.830188679245283
2015: 2.2108843537414966
2016: 1.48
2017: 1.513157894736842
2018: 2.122448979591837
2019: 2.8181818181818183
2020: 2.725806451612903
2021: 2.0

Number of channels in official music files:

Beach.j2b: 16
Bonus2.j2b: 9
Bonus3.j2b: 15
Boss1.j2b: 11
Boss2.j2b: 13
Carrotus.j2b: 12
Castle.j2b: 16
Colony.j2b: 16
Dang.j2b: 16
Diamond.j2b: 9
Ending.j2b: 16
Fastrack.j2b: 16
Freeze.j2b: 16
Funkyg.j2b: 9
Hell.j2b: 10
Intro.j2b: 16
Jungle.j2b: 10
Labrat.j2b: 13
Medivo.j2b: 16
Medivo2.j2b: 16
Menu.j2b: 10
Order.j2b: 11
Tubelec.j2b: 12
Water.j2b: 16
Whare.j2b: 16

For every official tileset, images of all the tiles that are not used in any official levels.

I'm pretending that every palette of a tileset has the same tile layout, which is not quite true. (For example, the Castle1 foreground pillar tiles aren't shown here because Castle1n uses those tiles as a green curtain, which is used in the level.) Easter is counted as a Carrotus palette. Multiplayer levels are included. A tile appearing as a frame in an animated tile counts as it being used even if the animated tile itself is never used, so that's why you see the (unanimated) bottoms of bonus signposts but not the (animated) tops. If the same tile image appears multiple times in a tileset, all but (potentially) the first appearance are hidden.

Poor eggplant

How many tilesets there are with sizes in each range of hundred tiles. We can see the most common size for a tileset is between 900 and 999 tiles, and it's hugely uncommon to have more than 1099 tiles, which is understandable because the maximum size for a 1.23-compatible tileset is 1020 tiles. Alternate palettes are counted separately, because they are separate .j2t files: for example there are fully seventeen different Improved Medivo palettes, all of which are 960 tiles.

September 2017 is an arbitrary cutoff date in that it's when I got stijn's J2O dump, but it's also pretty convenient in that MLLE's "Save Tileset Images" tool was added just a few months later, and that would confuse the higher number data.

Originally Posted by Violet CLM

Alternate palettes are counted separately, because they are separate .j2t files: for example there are fully seventeen different Improved Medivo palettes, all of which are 960 tiles.

Well, that dilutes the data set given its original purpose. Alternate palettes should probably be discarded wholesale.

The trouble is, I don't want to manually scour hundreds of tilesets looking for the ones that are probably alternate palettes

Are you able to check for whether or not the mask images are identical?

Oh that's a good thought. I'll try this again later.

Different official tilesets use (slightly) different colors for the sprite palette. Here's how they're grouped. For example, carrot1.j2t and easter99.j2t use the exact same colors, but carrot1n.j2t has different colors. What I'm calling "default" includes colon1, beach, beach2, jungle1, jungle2, inferno1, infernon, xmas1, xmas2, xmas3, town1, town2: almost every HH98/TSF tileset uses it (although hauntedh1 is very often totally unique), but even before then it was still the most common.

Generally, colors weren't made up from scratch each time, you see the same gradients recur across different tilesets in varying configurations. Here's the finer details:

4 variants for green
castle1, castle1n
psych1, psych2, medivo, medivo2, hauntedh1
carrot1, easter99
carrot1n, labrat1, labrat1n, labrat3, colon1, colon2, beach, beach2, diam1, diam2, tube, tubenite, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, town1, town2
4 variants for red
labrat1, labrat1n, labrat3, colon1, beach, beach2, diam1, diam2, tube, tubenite, jungle1, jungle2, inferno1, infernon, xmas1, xmas2, xmas3, town1, town2
castle1, castle1n, carrot1, carrot1n, colon2, damn1, damn2, easter99
psych1, psych2, hauntedh1
medivo, medivo2
6 variants for blue
hauntedh1
labrat1, labrat3, tube, tubenite
carrot1, carrot1n, labrat1n, colon1, colon2, beach, beach2, diam1, diam2, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, easter99, town1, town2
castle1, castle1n
psych1, psych2
medivo, medivo2
2 variants for orange
psych1, psych2, medivo, medivo2, hauntedh1
castle1, castle1n, carrot1, carrot1n, labrat1, labrat1n, labrat3, colon1, colon2, beach, beach2, diam1, diam2, tube, tubenite, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, easter99, town1, town2
7 variants for pink
castle1, castle1n
psych1, psych2
carrot1, colon1, colon2, beach, beach2, diam1, diam2, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, easter99, town1, town2
labrat1, labrat1n, labrat3, tube, tubenite
carrot1n
hauntedh1
medivo, medivo2
5 variants for yellow
medivo, medivo2, hauntedh1
psych1, psych2
carrot1n, labrat1, labrat3, tube, tubenite
carrot1, colon1, colon2, beach, beach2, diam1, diam2, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, easter99, town1, town2
castle1, castle1n, labrat1n
4 variants for brown
hauntedh1
psych1, psych2
castle1, castle1n, carrot1, carrot1n, labrat1, labrat1n, labrat3, colon1, colon2, beach, beach2, diam1, diam2, tube, tubenite, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, easter99, town1, town2
medivo, medivo2
4 variants for gray
hauntedh1
psych1, psych2
castle1, castle1n, carrot1, carrot1n, labrat1, labrat1n, labrat3, colon1, colon2, beach, beach2, diam1, diam2, tube, tubenite, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, easter99, town1, town2
medivo, medivo2
8 variants for other green
castle1, castle1n
psych1, psych2
labrat1n
labrat1, labrat3, tube, tubenite
carrot1, easter99
carrot1n, colon1, colon2, beach, beach2, diam1, diam2, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, town1, town2
medivo, medivo2
hauntedh1
7 variants for purple
castle1, castle1n, labrat1, labrat1n, labrat3, tube, tubenite
carrot1, carrot1n, colon1, colon2, beach, beach2, jungle1, jungle2, inferno1, infernon, damn1, damn2, xmas1, xmas2, xmas3, easter99, town1, town2
medivo, medivo2
diam2
psych1, psych2
diam1
hauntedh1

thank you, now i know.

and knowning is half the battle.

mieli = puhallettu