Within the last month both Mapbox and Etsy blocked my attempts to signup using a Proton Mail alias. How many services do you sign up for in recent years, on average? The practice is becoming incredibly common and more than likely you're just grandfathered in.
Etsy blocks my entire ISP (I know because my IP rotates almost daily) so I cannot even view their site at all, it just gives me a "you are blocked" page.
are you sure they're not just blacklisting protonmail vs. whitelisting known providers? ime a lot of sites block "temporary" or "anonymous" email providers