Domain validation TXT records are poor infosec hygiene. If used at all, those records should never include any hint as to what service they are intended for.
E.g. the record should NOT be:
example.com IN TXT "someservice.com-validation=029845yn0sidng0345randomnyosndgf03548yn"
instead, it should be something like
example.com IN TXT "029845yn0sidng0345randomnyosndgf03548yn"
Of course there will be multiple such records for different service providers. The service providers will just have to check all those (handful) of TXT records for the random assigned token in their database instead of pre-filtering them by the someservice.com-validation prefix.
As jelavich pointed out, there is also https://www.ietf.org/archive/id/draft-ietf-dnsop-domain-veri... which suggests another improvement on that. To avoid polluting the example.com name with tons of TXT records and to avoid problems with CNAME records and such, those records should be further down in the tree like
_validation_mv0395ng035.example.com IN TXT "029845yn0sidng0345randomnyosndgf03548yn"
The record name token "mv0395ng035" could either be another random number assigned to example.com by someservice.com they just put in their database. Or it could be something like HMAC(example.com, <common secret known to someservice.com>), so they don't have to save all those tokens. In any case, the check will be just one DNS lookup, one comparison and done. Quicker, equally easy and more privacy-preserving and infosec-hygienic.
E.g. the record should NOT be:
example.com IN TXT "someservice.com-validation=029845yn0sidng0345randomnyosndgf03548yn"
instead, it should be something like
example.com IN TXT "029845yn0sidng0345randomnyosndgf03548yn"
Of course there will be multiple such records for different service providers. The service providers will just have to check all those (handful) of TXT records for the random assigned token in their database instead of pre-filtering them by the someservice.com-validation prefix.
As jelavich pointed out, there is also https://www.ietf.org/archive/id/draft-ietf-dnsop-domain-veri... which suggests another improvement on that. To avoid polluting the example.com name with tons of TXT records and to avoid problems with CNAME records and such, those records should be further down in the tree like
_validation_mv0395ng035.example.com IN TXT "029845yn0sidng0345randomnyosndgf03548yn"
The record name token "mv0395ng035" could either be another random number assigned to example.com by someservice.com they just put in their database. Or it could be something like HMAC(example.com, <common secret known to someservice.com>), so they don't have to save all those tokens. In any case, the check will be just one DNS lookup, one comparison and done. Quicker, equally easy and more privacy-preserving and infosec-hygienic.