As Khaled said, this behaviour is intentional.
In versions past, the interpretation of codepoints between 128-255 was subject to the codepage settings in mIRC's font dialog. You are likely expecting those bytes to be intepreted as per the Windows-1252
"Western" codepage, as was the default in versions up to 6.35. This is no longer the case - mIRC has dropped support for the various codepages and has standardized around Unicode, in which the meanings of codepoints 128-255 are identical to those in the older Latin-1
codepage. To be clear here, it was by far never guaranteed that codepoints in the 128-255 range would appear the same for every user.
All of the characters you were using previously are still available in Unicode, you'll just need to find them and update your scripts accordingly. This may be as simple as copying/pasting them from unicode-aware sources. All IRC clients that have UTF-8 support enabled (which includes default mIRC installs from, I think, version 6.2) will be be able to view these characters.
A bigger problem, I think, is that users of non-Latinized languages may find that the new mIRC version does not 'correctly' display messages from friends who are using clients that do not encode characters 128-255 as UTF-8, and without the codepage settings they will be unable to fix this locally. Such clients include mIRC 6.35, which had outgoing UTF-8 encoding turned off by default.