Hi Michaël,
Undocumented or otherwise, we are loathe to remove features,
especially in a way that would propagate across all API versions. If
we were to consider removing the "deleted" parameter to GET
/v1/customers, we'd at the very least audit current users of it and
give appropriate notice.
That being said, the honest answer is that I had not personally been
aware of the existence of the deleted parameter to GET /v1/customers.
So, that was a gap in my own knowledge when addressing your question
initially -- apologies there.
With regard to plans, the ID caveats are much, much stronger. We
still allow user-specified plan IDs, and we still allow "recycling" of
plan IDs (delete plan with id foo, then create new plan with id foo).
And, anecdotally, I do think that we have plenty of people re-using
plan ids. So the semantics of listing deleted plans are arguably more
problematic.
Since you will always see plans that were used by querying customers
and/or invoices, do you have a bonafide need to list out the deleted
plans? We can consider it, but again, with the ID recycling issue, it
has so many caveats that I wonder if releasing it does more harm than
good...
Jim