When viewing the Sprint Backlog board and Work Details, is there a way (possibly via extension?) to mark a given user differently to show they are actually marked as Off for the given day?
From my screenshot, the bottom user is out for this whole week, but you don't really know that until clicking over to the Capacity tab to view days off.
I'm afraid that there are no supported extensions from Microsoft doc at present.
You may submit a suggestion at website below:
UPDATE
You can use Capacities-Get
REST API to get user's daysOff information. And you can build by yourself. Please Refer to doc: Capacities-Get