Thanks for the hint!
I was referring to the version listed in Google's webstore (https://chrome.google.com/webstore/detail/tampermonkey/dhdgffkkebhmkfjojejmpbldmpobfkfo) which I was using before and which is still blacklisted for whatever reason. I would prefer to stay with that version, which got updated much more frequently in the past. As I wrote before--every other Chromium-based browser allows it. I cannot see what makes it so especially "malicious" when used with Opera.
I've been using that version in Vivaldi since Opera started blacklisting Tampermonkey (actually, I only used Vivaldi since that day) and haven't noticed any negative impact. Maybe I'll just keep using Vivaldi, it has all the features Opera has. It only feels a little less responsive than Opera does in my everyday use.