I’ve seen L brackets used a lot but I don’t recall any previous ones attached to the side walls of the game. That said this kind of mounting isn’t ideal as it interferes with raising the playfield limits the mounting options to things that are easily detached.

This is the main problem in my opinion. There has been talk of a filter for a while and I haven't seen anything that works. Even if a of some kind were available I can imagine you would still see the refection it would just look less like a candy cain or chiclets or whatever. I realize some aren't bothered by it, but in my opinion it looks awful in person and even worse on streams, especially when there are polished metal or plastic pieces near the edge of the play field. The cheaper solution that avoids this problem is just to add spot lights to games that need them, lighting under the apron or on the back board as needed to address dark areas. For streaming use carefully placed external lights and/or a HDR camera that can capture detail in bright and dark areas.