Bad execution plan when having partial index and big In-clause in Postgres
This will be fixed in version 12, which will be released soon.

I think the summary here is that we are only willing to do so much work to try to prove a partial index can be used, because all queries have to go through that work even if they don't end up using the partial index.  In this change, they just found a more efficient way to do that work in this particular case, and so no longer impose the 100-element limit to it.