Side inserters are insanely powerful and I use them all the time for compact and fun builds, but I think it would be better balanced if they were available only as a technology.

Side Inserters [Auto + Logistic + 30s] x 250
Requires: Advanced Logistics, Fast Inserters
Free inserts to pick up items both from behind and the sides, and place items on either side of a belt.

It's worth considering

On the other hand, adding technologies could interfere with mod compatibility.

Locking it behind a technology is kind of an intuitive suggestion, but if you think about it a bit then it stops making sense.

This quarter-turn should simply have been in the base game. Maybe choosing which side of the belt the product goes on could be separated into a research but even that itself is also a bit daft, don't you agree?

The whole reason I think both these options should be vanilla is because the arm has the reach for it. So why add that dumb limitation to an inserter? In fact, it almost looks like the arm cannot even reach the outside of the belt (where it usually drops an item), which is funny if you think about it. Looks a bit like the factorio devs had it in mind to push the idea of 1 belt for input&output but dropped it halfway down the drawing board in the meeting room.

If I ever did add technologies it would only be an option and it would be disabled by default

Belt lanes are a feature that feels a little incomplete. While it's better now that splitters have filters it still feels like the vanilla toolset to deal with them is lacking