On a different note, adding a file on webserver is one of the ACME methods (HTTP-01) to get a SSL/TLS certificate so it is indeed considered as possession method in real world already
You're missing that HTTP-01 challenge grants you no ability beyond what the check has demonstrated, i.e. you have proven that you're able to serve random file from a webserver, so the grant is to allow you to serve them via TLS connection.
There are no comparable _technical_ proof-of-registration methods because all of them would require actual access to registrar control panel and be outright silly ('point the domain to a random nameserver').
So no, proper registrars never use webserver control as means to prove identity or ownership.
> you have proven that you're able to serve random file from a webserver, so the grant is to allow you to serve them via TLS connection.
Its digression from the original topic, still, it goes way far than that, these certificates are signed by a CA that the client devices trusts by the virtue of root certificates installed on the device. If I can some how obtain the SSL/TLS certificate for google then thats a very big deal.
There are no comparable _technical_ proof-of-registration methods because all of them would require actual access to registrar control panel and be outright silly ('point the domain to a random nameserver').
So no, proper registrars never use webserver control as means to prove identity or ownership.