Reported at https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)#Search_totals_capped_at_10,000.
As part of the Elasticsearch 7 upgrade, search queries now "early stop" so they only return a total count up to the limit of 10k (which has always been the limit). This means that users can't see how many results total there are (the 10k search limit itself is not new).
On IRC @EBernhardson and @RKemper said it's possible to set a flag so it'll return the correct total count, but that there may be performance implications in doing so:
18:33:59 <+ebernhardson> yea, we can probably turn it back on (it's a boolean in the search query), and evaluate if it makes sense to save perf / latency in specific situations 18:34:11 <+ryankemper> Whether that use case is worth the extra CPU for us to change the behavior back is up for debate, but might not be a bad idea to flip that back so we're not surprising users with the behavior change 18:36:19 <+ryankemper> (Personally I'm leaning towards keeping the old count behavior but I definitely don't have a perfect grasp of how relevant the perf implications are) 18:36:43 <+ebernhardson> probably not worse than what we had yesterday without the early-stop :)