We currently don't require a body on POST /rooms/{roomId}/receipt/{receiptType}/{eventId}.

Initially, this seems to have been a mistake, but since #10531 [v1.40.0rc2], it's a workaround because Element Android relied on that behaviour (which we broke by actually using the body for hidden read receipts in #10413 [v1.40.0rc1]).

Blockers:

• Yell in Client Developers room now
• Element Android doesn't send a JSON body for POST /rooms/{roomId}/receipt/{receiptType}/{eventId} element-hq/element-android#3789
• Figure out how long Element Android needs to propagate, set target removal date
  Minimum of four weeks following release. Revisit in October.
• Wait x time for a fixed Android release to propagate

Eventually, we should return to enforcing the spec.