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):