They have ignored this request for almost 10 years. https://protonmail.uservoice.com/forums/935538-accounts-payments/suggestions/33188971-accept-monero-for-payment

Like 3 or 4 years ago, Seth said they might soon accept Monero, but they didn't either. Proton is not too bad, to be sure, but it's been really questionable in various ways too... Better late than never, though.

Cf. elsewhere https://l.opnxng.com/r/Monero/comments/1lmuhfi/proton_to_add_monero_support_with_1000_signatures/

Yeah, I saw that elsewhere. I have a hard time believing this petition is legit.

I do wish they would accept Monero, but the longer they don't, the less I trust them.