> I want to filter all to:`X.X@domain.tld`

You can filter based on the to: field, yes.

For many years I’ve been creating filters on free Gmail for to:, from:, subject:, etc. I set them up on desktop web.

Perhaps there is something more specific you’re trying to do?

> a major feature gap that Gmail (paid or free) cannot create filters on headers.

You can create filters on header fields like from:, to:, and subject:, so I am guessing you mean something different than “cannot create filters on headers”?

I think he refers to mail headers. Those are normally hidden away from you, at least on a PC browser you can see them:

1. Open the specific email.

2. Click the three vertical dots (More options icon) next to the "Reply" button.

3. Select "Show original" from the dropdown menu.