I was aware that the Classic-style project boards were deprecated, because it had been communicated in the UI for quite a while, but kinda assumed that they would just become read-only, not get deleted.

I had a few such boards both at user- and repo-level which are apparently now gone, and that's annoying because I don't entirely remember how many there were and what all was on them. But it was largely redundant information in my case so no real damage done.