Build 55933 for 11.0.0 is now live and should have the fixes to get the extra mouse buttons working again.
As a reminder, you need the CVar enableWowMouse set to 1 for the extra mouse buttons to work.
e.g.
SET enableWowMouse "1"
in the WTF/Config.wtf config file
or run
/console enableWowMouse 1
in game