Outdated because it has been replaced by a better feature which has no equivalent anywhere else on the keyboard or mouse on other systems - as far as I can tell, not even Mac with its Unix heritage uses MMB paste.

Ctrl+C, Ctrl+V is superior. The shortcuts are easily accessible and you can use it to select and replace a specific part in the middle of text. If you try that with MMB-pasting, you will just overwrite your clipboard with the part of the text you want to replace, due to the selection-to-clipboard feature.

The answers from your "quick Google" only provided solutions for specific programs, solutions with self-admitted performance issues and a solution for X11 when I'm on Wayland. At a glance, the latter two look pretty hacky too.

Lastly, stop putting words in my mouth. I have said my issue was that the "Middle mouse button is paste instead of autoscroll with seemingly no global way to change it" and "I just wish there was an OPTION to change them". It is YOU who is forcing your defaults on me, without even a consideration that my preference may also be valid.

While I don't agree the parent posters view on the middle mouse, if you actually read the link you posted you would find that none of the answers suggested there actually solve the problem and in fact offer pretty convincing evidence that reproducing the desired behaviour cannot be reasonably done in Linux. Dismissively saying "you're holding it wrong" or posting links to irrelevant forum posts while telling people to RTFM isn't a winning argument for Linux. If you're going to tell someone to RTFM at least make sure you're pointing them to the correct M that contains the answer they're looking for.