While we did use phone verification in the past, this is not the case any longer. Phone numbers were stored in the same way as the email addresses, so, again, we have no way to derive them back from the hash.
This isn't true in practice. It's not hard to build a big list of ~every email address (give or take), and have a GPU churn through them all until you get a match.
If you've ever received a spam email, your email address is on such a list.
If equality-check is required to prevent e-mail reuse by spammers then argon2id with static salt rotated every few months will be reasonably strong too.
Of course I have no idea if any of this is implemented or it's just sha256(email). Just replying to the question of general feasibility.
I've no reason to doubt this but brute-force cracking a hash known to be from a phone number would likely be pretty trivial.
Fwiw, I use protonmail and trust it more than most other services. But my threat model doesn't involve technically capable adversaries directly targeting me, certainly not ones that could compel protonmail to divulge phone number hashes.
Unfortunately, it can and has been abused.
About the only way to even vaguely keep your email private is to use a self hosted server with GPG keys. And any lapse on security updates for that thing and you could be compromised almost immediately.
Beyond that I cannot think of anything more one could do.
I have always treated email as something to travels in the clear. My current provider (Fastmail) is compromised by authority. The Australian Privacy Act 1988 by being based in Australia and it gets caught up by PRISM as the servers are run out of New York.
If you continued using the account only through Tor, there wouldn't be any traceable info.
It did then prompt me to add an email and/or phone number as recovery methods, but that step was skippable.
Protonmail can still be the best choice for a pseudonymous mail service so long as it's combined with diligent, consistent IP address obfuscation. Protonmail will continue to allow logins and new account creations over Tor. All the major free email providers have long since disallowed new signups over Tor, and most have some form of degraded user experience when logging in over Tor, if they allow it at all. Small, niche email providers appear and disappear so often that relying on them still to exist even a few months into the future is a big gamble. Hosting one's own email requires payment of some type to the hosting provider, so it is not anonymous. Other privacy-oriented free email providers, such as riseup, will do exactly what protonmail did, because if they refuse, their only option is to go the way of lavabit.
Get one from your neighborhood coffee shop Wi-Fi, and pay cash for your coffee.
Also make sure to avoid CCTV...