Looks like the forum has got another update, judging by some new symbols, but the search widget is no longer there, Ctrl + F not working like it used to either.
The only way I can access it right now is via an URL:
https://us.forums.blizzard.com/en/hearthstone/search
.
Using Firefox 136.0.2, in case that matters.
[UPD]
It looks like the search widget is there, but it’s invisible. 
If you click on the place where it used to be, the search box pops up.
Ctrl + F sometimes activates the forum search — or rather topic search in some topics, but this behaviour seems sporadic atm.