Domain verification should typically be a one-time or at least rare event. You shouldn't have to keep the txt records after the verification is completed.
No, domain validation should be frequent, so that revoking authorization can take effect quickly, which is particularly important if the domain changes ownership.
It should be one-time, yes. Maybe it shouldn't be rare though. But your point still stands as the TXT records should be ephemeral. So I don't think this deserves the downvotes.
At least ACME's DNS challenge protocol is designed this way.
> The client SHOULD de-provision the resource record(s) provisioned for this challenge once the challenge is complete, i.e., once the "status" field of the challenge has the value "valid" or "invalid".