Yes, in many places, mIRC defaults to having a case-insensitive search/match.
Generally, you're not going to get a case-insensitive search/match unless a specific operator or identifier or switch has been created for it. i.e. 'iswmcs' or $istokcs.
Also, the 's' and 'w' switches for $read are case-insensitive, with the only way for a case-insensitive search is using the 'r' regex switch.
Likewise, my script here https://mircscripts.info/?page=project&id=5kLCC
can search all scripts/aliases for something, but it uses /filter to search for matches, and the only way that /filter can do a case-insensitive match is when using the -g regex switch. so for your example "/srch -r * X" would find all lines containing a capital X.
A case-sensitive switch for $read and /filter would be useful, because many users are not familiar with regex, and even for those who are, some searches can be more complicated in regex than in wildcard lingo