@Yuri What was the failure case motivating the issue? Was there a port or software that otherwise has issues whilst building? Additional info/refs would be great.
This appears to be a dupe of bug 255374, where the relevent headers were added, and MFC'd to stable/13 (but not stable/12).
We'll re-open that issue requesting a decision on what to do about 12.x
*** This bug has been marked as a duplicate of bug 255374 ***
(In reply to Kubilay Kocak from comment #1)
> What was the failure case motivating the issue?
I just manually tried '#include <format>'.