2. Do you remember the nature of the legacy ircd behavior that motivated the mirc behavior here? It seems to me that even if some ircds were sending an additional parameter here, it would be correct to take the final parameter (according to the section 2.3 grammar) and treat it as the nicklist.
I'm afraid I cannot. mIRC has been in development for over 25 years, so there is a vast amount of code that has been tweaked over decades that caters for a wide variety of ircds and for issues reported by users during this time.
While it is possible that some of the ircds catered for no longer exist, this particular change has been in place for some decades and has worked with all ircds.
My guess is that most ircds specify the colon because it is a clear demarcator, making it obvious to the client where certain data is located in a server message. mIRC looks for the colon in quite a few server messages. Again, this is probably due to the way some ircds use slightly different formats for server messages or use the same numerics for completely different purposes.
