There really are a lot of clients out there that have only tested against Let's Encrypt.
Yeah, I was going to say the same thing. It'd also be great if there were a client of some sort analogous to Pebble, so that ACME server implementations can test against some, uh, looser interpretations of the standards that are out there.
It might be nice if ACME profiles could help with this. I don't even know as one needs a specific "strict" profile, but just clients that pass a profile at all are hopefully staying more up-to-date, and so requests that specify a profile probably implies that the client won't get surprised by asynchronous order finalization, case-sensitive handling of keys, requiring POST-as-GET, and so forth. And maybe just a little more robust than changing behavior based on user-agent. Though I totally get that trying to support both "strict" and "loose" clients might expand your test matrixes more than desired, too.