[vages]

I tested this, and you're right: Their Tor service runs the (in my opinion nearly as usable) older version.

It's plausible that Proton does not care about their Tor service, but there may be another reason: The new version relies more on Javascript code than their old version, and a Tor user is more likely to browse with scripts disabled than a regular user. Proton may be holding back the rollout of the newer version until they have tested it more without Javascript. This is only a hypothesis, and I came up with it just now; take it for what it is.

[uncoder0]

Tor + Scripts is not good. You're probably right on why there isn't a newer version on their onion service.