RFC 2812 specifies in §1.3 that “Channels names are strings (beginning
with a '&', '#', '+' or '!' character) of length up to fifty (50)
characters.”
We previously only allowed the first two prefix characters, or any other
one if the server overrides the list with ISUPPORT.
It’s now easier to use. The distinction between a nick and a channel name is
based on the first character (by default it's '#' and '&'). The user doesn’t
have to worry about which separator to use anymore.
fix#3066