What do we mark after a fire emergency, on Openstreetmap?

Destroyed buildings: Use a lifecycle prefix on the building=* tag, as InsertUser says.

Amenities (,shops, etc) co-tagged directly on the building way: Delete the amenity-specific tags, optionally add a lifecycle prefix on the main tag (eg destroyed:amenity=restaurant). Consider tagging old_name=* -- if people still refer to the site by the previous name.

Amenities etc mapped as a node inside a destroyed building: Delete node or use lifecycle prefix on the main tag.

(Possible edge case: Some amenities, churches for example, may continue to function in ad-hoc accommodations on the previous site.)

~~~

As far as I know, there's no standard way to mark a temporary closure for an entire area, but you can close the roads in that area...

Closed road you expect to reopen within a few weeks: Don't change road access, possibly add a note or fixme explaining the temporary closure and asking for a resurvey to confirm the re-opening.

Closed road you expect to reopen after several weeks: Add a conditional access tag like conditional:access=no @ (2020 Sep 1-2020 Nov 15). This is an imperfect solution for two reasons -- First, data consumers will need to specifically be looking for the conditional access tag, and many of them do not. Second, you have to choose a re-opening date... but don't worry if it's not exactly right, you can remove it if you overestimated the length of the closure and revise it if you underestimated.

Indefinitely closed road: Tag access=no and add a fixme asking for resurvey.