First, this is trivially incorrect: there's also !blue, !red, !green, !yellow, and most importantly !k.

The historic reason for this is that these are a very narrow class of commands that are usable by any client (but only by clients, oddly). Even if you're not running JJ2+, as long as the server is running JJ2+, you can use !swap and !k and the rest... come to think of it, for maximum consistency, shouldn't /login be !login?

I worry that it would be confusing for clients to not know whether they're supposed to type !swap or /swap depending on the version of the server they're in, which they shouldn't otherwise need to care about so much. (And if we changed !k too, this would be much worse, because the "respawn" esc menu option would suddenly work only in some servers but not in others.) If we were in an age where all servers and clients reliably ran the same version of JJ2+, that wouldn't be so bad, but despite our efforts with the autoupdater, that's just not how people behave in real life.