Right now helloworld.letsencrypt.org uses an older certificate issued from the X1 intermediate. That certificate expires this weekend.
If it’s replaced with a certificate from the DST-signed X3 intermediate it will still work in people’s browsers but is likely to stall the application for inclusion into the Mozilla trust store (and perhaps other trust stores) because it doesn’t chain to the ISRG root and no other example has been cited.
If it’s not replaced obviously it expires, and that too would likely stall the application for inclusion.
The Right Thing™ here is to undertake the ceremony to sign X3 and X4. It sounds like that’s not going to happen, at least any time soon. So the closest alternative is to (manually?) issue for helloworld.letsencrypt.org with the obsoleted X1 intermediate again. That’s unfortunate because manual issuance sort of undermines the point of ACME, but it’s probably the least worst option.
Or am I missing something?