It would be nice if they had a tor presence so there would be no need to traverse an exit node. A lot of other privacy-forward sites do exactly this.
https://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7... is the URL that people should be using with Tor if they are concerned with being traced.
Using Tor to access a regular web site, ie: through an exit node, is (nearly) no different than using a proxy. Just assume all exit nodes are "naughty" and do your thing accordingly.
So Proton users who like to connect via Tor onion services have been faced with a choice of either staying on the old version of ProtonMail, or giving up on connecting via a Tor onion service. It definitely leaves the impression that they don't care much about Tor anymore, or that it's at best an afterthought for them.
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.
Perhaps there is a greater reliance on scripts in the new version, but this makes it seem more likely that they've abandoned the Tor version.