I first saw this in Indonesia on my first trip to Indonesia in the late 1990s. It was when we went to a "fancy" hotel in Surabaya for a bacon and egg breakfast instead of a local restaurant. That was ++.
Then we went to Singapore and it was +++ FFS! Again, it was in more "fancy" places. Hawker centres etc didn't do that nonsense at that time.
When I first came to Thailand in 1997 I went to places like Koh Samui, the "Koh San Road" and Pattaya and I rarely saw the ++ nonsense. That was both at local restaurants and foreign restaurants. I ate mostly at "budget" places and just assumed that the practice was exclusive to "higher end" places that I couldn't afford 😂
I don't know where the practice originated in SEA but I've always gotten the impression that it came from the American hotel industry. I've no evidence to back that up, it's just a gut feeling.
Now that I have more funds than in my backpacking days I don't have to stick to "budget" restaurants. I've noticed that the ++ in Bangkok isn't limited to the fancy hotel chains, it has spread to lots of other places as you've noted.
I wouldn't say that the vast majority of restaurants do this because Bangkok is a huge place and they don't do it in "normal" Thai restaurants. However it is quite common in places like Sukhumvit and Silom.
I too despise the practice however there's sweet FA that I can do about it so you just have to suck it up and either pay the extra 17.7% or stick to restaurants that don't try to trick their customers.