I got a subscription last year and I don’t have a memory of seeing an advertisement at https://mail.proton.me/u/0/inbox after that, and I didn’t see one just now when I checked.

If you want to use an email service without seeing advertisements, maybe you should pay for a subscription starting from https://proton.me/pricing or https://www.privacyguides.org/en/email/ or maybe instead you should consider something like https://www.lifewire.com/best-free-email-accounts-1356641