Like most features that Google releases, I believe that they use them to train their algorithms and once they get enough human input on a feature from people using it, they remove the feature and turn it over to the machine. We see it on the SEO side all the time. Release a feature, call it a ranking factor, thousands of SEOs jump all over it. The algo learns and then the features importance is removed. NoFollow links are a great example of this.

Commercial pressure seems the most likely to me

How would such "pressure" look in practice? Seems to me that every industry on the planet is at Google's mercy, not the other way round.

How many google ads are on github or stack overflow compared to the other pages you find? They know exactly what every little change to the algorithm does to their bottom line.

Search doesn't have to concern itself with AdSense ads, search ads still make up something like %85 of Alphabet's total revenue

The worst sites a.k.a. those a user might want to block are probably also bringing Google disproportionally large amount of income is my guess.

I'd put my money on performance costs.

Personal remove lists basically turn each query from each person with such a list into a completely distinct query, which breaks caching on multiple levels. If a few people are using such a feature or if people sometimes add a couple -sites to a query, no big deal, but if enough people used it with basically unique site lists, the performance degradation would probably make a rollback of the feature inevitable.

I can imagine it was also a handy tool to DoS a site.

It was a tool to remove a site from your own results. If you searched and saw a site you didn't like, you could click to no longer get the results from that site. It wouldn't affect what anyone else saw anywhere.

That's a good point. I guess bad actors (exactly the same ones that publish those shit-sites) can abuse it to block the actual sites.

Not a problem as long as it only affects the user themself?

My guess is some ux head exploded when they realized someone might blacklist a site they wanted. Or maybe the one who made it got promoted and no one stepped up to maintain it?

Adding a minus sign in front of a site: specifier still works for me (eg "google -site:google.com"). Works on DDG, too.