Fastmail (and I assume others) let you create "masked emails" (read: ephemeral aliases). You can do it with your own domain too. Couldn't live without it.
Is it the same feature offered by SimpleLogin from Protonmail?
Arguably. SimpleLogin is an integration with Proton, not a Proton feature. Fastmail integrates its feature with 1Password and Bitwarden without the need for additional add-ons, I don't know how SimpleLogin works but it feels more akin to Mozilla Relay and whatever else exists out there to create ephemeral addresses - but ultimately the feature integration might lead to the same use case.
I have been using the alias feature for years now with Fastmail. Deleting an alias is hands-down the quickest and most convenient way to unsubscribe from whatever drivel a company ends up sending my way care of their slimey marketing vendor.