Does FLASHBACK QUERY work across incarnations or after a Data Guard failover?

Short answer: yes.

Let’s just see it in action.

First, I have a Data Guard configuration in place. On the primary database, the current incarnation has a single parent (the template from which it has been created):

Just to make room for some undo, I increase the undo_retention. On a PDB, that requires LOCAL UNDO to be configured (I hope it’s the default everywhere nowadays).

Then, I update some data to test flashback query:

At this point, I can see the current data, and the data as it was 1 hour ago:

Now, I kill the primary database and fail over to the standby database:

After connecting to the new primary, I can see the new incarnation due to the open resetlogs after the failover.

And I can still query the data as of a previous timestamp:

Or flash back the table, if required:

So yes, that works. The caveat is still that you need to retain enough data in the undo tablespace to rebuild the rows in their previous state.