Most likely most downstreams have not yet substantially adopted either standard. Below is what I found after a brief search through the above sources, project webpages and other sources.
- Proton Mail: involved in crypto refresh; uses and maintains OpenPGP.js
- Thunderbird: uses RNP but is freezing implementation of new OpenPGP features
- Mailvelope: ?
- OpenKeychain: ?
- GnuPG: involved in LibrePGP
- Hockeypuck: the draft standard for OpenPGP keyservers has some support for v6 keys but imposes limits in consideration of clients that do not support v6 keys
keys.openpgp.org: its governance board includes people from Proton and Sequoia- OpenPGP.js: supports crypto refresh; maintained by Proton Mail
- RNP: openly supports LibrePGP
- Sequoia PGP: involved in crypto refresh
I found this article by a keyserver operator that discusses technical differences, describes the conflict as personal rather than technical, and proposes a way forward that hopes to achieve harmony between v5 and v6.
Yes, some implementations do their own quirky things, others simply don’t implement OpenPGP in full.