I know MONITOR is supported on both Freenode and EFNet, usually regarded as two of the most popular networks. WATCH is supported by neither.

EFNet is Ratbox, Comstud, based on Hybrid IRCds.
Freenode is IRCD-Seven, based on Charybdis Atheme.

It's more than a little popular. WATCH will not be implemented in these.
(first hand statement when asking their developers)

Well. At least I won lunch.
Good philosophy, see good in bad, I like!