Thanks for testing the installer.
In %programfiles(x86)%\mIRC the dated "backups" folder contains the contents of the installation folder (mirc.exe, readme.txt) and the defaults folder. These do not seem like useful things to back up.
These are indeed critical files to back up, in case a user has issues with a newer version of mIRC and wants to revert to their older vesion. This was actually one of the most common requests/complaints long ago - not backing up these files.
The backup folder %appdata%\mIRC\backups\20201125-232521 contains only the file cacert.pem. The customized $mircdir\mIRC.ini isn't backed up anywhere.
Did you perform a full or upgrade install? In an upgrade install, cacert.pem is the only file that is replaced in the user data folder, so it is backed up.
The backup feature only backs up files that are replaced during the install. That is the purpose of the backup feature of the installer.
If you are testing the installer, please be sure to test it with all of the basic options, eg. "full", "upgrade", "portable", etc. These could all have an effect on how the backup works.