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/