I would agree this is a very important feature. I would assume that allowing unlimited (large)number of addressees per domain might be challenging due to encryption (as each address has a set of associated keys). This solution, however, could work without the keys.

There are dublicate to this request. If they could be bundled, this feature would move up in the wanted features list :-)
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/41522041-reply-from-catch-all-address
https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/36214159-send-from-any-address-of-custom-domains-if-catch-a

what @thomasedward said.
Critical for me as well to migrate one of my domains to proton.

This would be amazing if it was possible. Especially if it just did it automatically, if there was a flag to turn on the option. "Always reply with envelope recipient address".

This is CRITICAL.

I have a custom domain specifically so I can give each merchant I'm dealing with a unique address. For example let's say I was doing business with McDonalds, I'd give them the email address mcdonalds@[mydomain].com, then the catch-all for [mydomain] would receive it. But now if I want to send an email in reply, it will come from info@[mydomain].com. I need to be able to set the "from" field manually so that it instead looks like the email was sent from mcdonalds@[mydomain].com.

I do not want to have to set up a separate address for every single merchant - that's totally unworkable.

It's best to leave the catchall turned off. With catchall turned on, if someone sends an e-mail to a non-existent address, it looks to them like it went through to the intended recipient. Leave it turned off so the sender gets back a response telling them that they made a mistake with the address.

Second this, ability to be able to send from any catchall from custom domain or ability to add unlimited amount of custom domain usernames seems vital.