Instead of having identifiers like $modespl, $prefix, $chantypes etc., that rely on the RPL_ISUPPORT 005 reply from the IRC server, why not just implement $isupport(<parm>), which would return any of the many parameters coming from it?
mIRC could provide $isupport(MODESPL) instead of $modespl, $isupport(N) could return the Nth received parameter and so on.

These identifiers seem just like the deprecated $opnick(), $nopnick(), $vnick() etc., all that ended up becoming only $nick().

Also, before going to another point, firstly I must ask so I won't assume things, is there any known reason for not having (an) identifier(s) refering to all the RPL_ISUPPORT parameters?
The point is, mIRC doesn't offer identifiers to many of them (like KNOCK, WATCH, *LEN parameters etc). I know it's easy to catch them using a raw event, but mIRC already catches some of them, so I'd rather have them all built-in instead of only part of them. $isupport() could handle all of them and any new ones in the future, if they come.

* cold edits his posts 24/7