Proton.me: “We will be adding Monero as a payment option for our VPN plans by the end of Summer 2025. Source: https://x.com/davidgpeterson/status/1941500514803548636”
UPDATE: they support it only via 3rd party authorized provider: https://digitalgoods.proxysto.re/en/brand/proton
Upvote related feature requests: https://protonmail.uservoice.com/search?filter=merged&query=Monero
Proton staff says that one can purchase through authorized proxy which accepts XMR:
https://digitalgoods.proxysto.re/en/brand/proton