Yes, Google used to have this feature. I was probably one of the tiny percentage of the users who used it. My guess is it's one of those features where very few people used it. So the cost of maintaining a feature like this on a scale like Google search with tiny percentage of users is not worth it.

I think it was a stop gap solution because Google was overrun with spam sites, like Expert Village, before Google did the Panda update.

https://en.wikipedia.org/wiki/Google_Panda

Yep! It was called “personal blocklist (by google)”.

I don’t think anyone was maintaining it. So when the SERP changed its layout, the extension broke and Google removed it :(