I disagree about it not being in Accessibility.

Accessibility isn't just for people with major accessibility challenges. It is full of things that many people would benefit from using, especially as we get older...

I realise that, I started to use the stuff in windows, like making the pointer brighter and larger, strange as it is something I have not had to do on my Mac.
But I think since getting rid of transparency or reducing it is customisation, then it should be in appearance. Maybe they will put a slider in appearance to reduce the transparency, we will see.

The Reduce Transparency option in Accessibility isn't a new thing by the way, it has been there for a long time (I think maybe some 2014).

So it is, not really looked.
Still need to have a slider if they are increasing the transparency.

Well, I have installed it, nothing that exciting about it, the transparency is not too bad, well not for me, not really had a good look around.

For me, it is just a different look, I may use some of the new features, the new spotlight looks interesting.