I think it was introduced in the 3.7 server.

Depends on server level, not client level though; I've still got some WinNT
clients running 3.1.0.6, and they have MAXNUMMP set (although as I recall
there was an issue when I first moved to that server level, that clients
which had NEVER had their MAXNUMMP set, would not behave as they should by
default, until I set it explicitly; or clients that had MAXNUMMP that
defaulted to 0 didn't behave the same as ones that were explicitly set to 0,
or something like that.  Anyway, I can't check now, because I remember I did
a global update node * maxnummp=1 to clean it all up.

But that was a long time and several sever version ago.  Just thought I
would mention it...