See if this patch can solve that issue for you. I'm still using the early June 2023, thus previous Steam's UI and with it I can still PM my friends without any issues.

A few days ago I was forced to resuse my "Steam's UI downgrade link" (i.e., *.lnk) file which is the following: "C:\Games\Steam\steam.exe -forcesteamupdate -forcepackagedownload -overridepackageurl https://web.archive.org/web/20230702125953if_/media.steampowered.com/client -exitsteam" to be able to "Verify game files integrity" for Carmageddon: Max Damage because the former Steam UI (User Interface) wasn't allowing me to do it properly. I also had to redo my usual patching to obtain a working friend's list and also make and/or place that "steam.cfg" file back to prevent Steam from trying to update itself.