There have been a large number of threads suggesting a 64 bit version of mIRC. One such can be found here
and another can be found here
The general consensus is that at this time it's not worth it due to the small amount of people that actually use a 64bit OS. 16-bit support did not stop on mIRC until 32-bit had almost completely taken over the market. A whole bunch of technical arguments however, can be found in those threads and others, you might like to use the Search
feature. Expand to 'All Forums' and 'All Posts' for best results.
Do note this is not a post saying "There will be no 64 bit" - Khaled does not generally say what will be in the next, or future, versions of mIRC. Nobody, including myself, knows what will be in the future versions of mIRC except Khaled so basically it's a matter of waiting!