Hello, I disagree strongly and entirely:
In many programming languages or when editing a configuration file, the "#" symbol is commonly used to add single line comments between lines of code.
mIRC is using the famous C comment /* */, which almost all major programming language i can think of, support, C C++ javascript mIRC C#, probably a lot more. # seems to be used in python, not sure else where, seems to me like # for commenting out is way less used than /* */ (plus, they do have the requirement that /* start the line in mIRC, same as # then). So right at the start, there's no such thing as # being commonly used compared to mIRC's way of commenting with /* */
Alternatively equals with the same semicolon "#" = ";". So that, in this way, this commenting can be similar to coding as in other languages, and so that it cannot cause similar errors.
This may be more convenient for those who are used to commenting their code in this way, and it simply stands out much better against the background of large multi-line code.
And in particular, it would be more convenient for me.
Here you repeat that it would be similar but it's the same demonstration as above, mIRC is already being similar to most languages, adding another character would actually create lot of issues, it wouldn't stop causing some hypothetical issues you're not even mentioning to stop being there.
All in all it would be handy for you to have it, but only you so far.
Now, I am against this suggestion for any character, but it has been demonstrated on multiple sides that # will break backward compatibility, either because of groups or because of # being treated as $chan (so if $chan has a value, it should execute that alias)
Even if all programming language where using # and mIRC weren't and already had something others than /* */ or ; or rem, I'd still be against it because the functionality would be already there, it's not offering anything, you're just bloating the language
