Middle click paste is a very useful feature for a lot of people, but new linux users are not those people.
I personally switched two years ago, and got several people I know to switch too. Everyone I know who switched (including me) was confused by middle click paste.
It’s a hard to intuitively understand action (took me several months until I understood it took the selection for some reason) that is very easy to trigger accidentally, and that duplicates existing functionality.
The people who like it already know it exists, and could just toggle it on.
Of course, on distros not aimed at beginners, like say, debian, it should remain the default.
I’ve been using Linux off and on for decades, full time for over a year now, and only now learned this was a thing after I distro hopped to CachyOS and kept accidentally middle clicking on my laptop’s trackpad and getting shit pasted everywhere. I had to look up how to disable it because it was so annoying. I generally only middle click in web browsers for auto scroll and that’s extremely rare.