This may be something to do with the recent downtime and relocation of SE servers to Colorado; I think that was around the time I started noticing problems.
Chat search is broken, apparently displaying only results up to ~2015.
I guarantee there should be more results for searches like this and this, but nothing is displaying after around autumn 2015. This is very annoying, because there are a lot of things said in chat that I'd like to find again (for example, right now I want to find the mini-reviews of the Belgariad series which I posted in chat in order to turn them into a published mini-review for a blog).
Not a duplicate of Chat search is broken again - that one's from 5 years ago and presumably a different underlying issue.