> You could use 404 but return a custom error body and demand that clients check for a correct error body. This is asking for trouble from lazy client programmers. It might or might not be "your fault" when clients see eventually inconsistent data, but the support calls they send you will be real.
Make sure that your 404 responses were always documented, then tell them to RTFM.
The problem is that the support call comes in as "your DELETE call isn't actually deleting". Sure it's not your fault, but it imposes a cost on you to investigate. And of course the first time you go directly to RTFM without checking will be the time it actually is your bug.
404 is special because it's so incredibly common. Why take the risk? There are other perfectly good error codes that - in practice - don't have this issue.
> You could use 404 but return a custom error body and demand that clients check for a correct error body. This is asking for trouble from lazy client programmers. It might or might not be "your fault" when clients see eventually inconsistent data, but the support calls they send you will be real.
Make sure that your 404 responses were always documented, then tell them to RTFM.