I think it's only caching that is a factor here. It's certainly possible that DNS resolvers could refuse to cache, or somehow restrict caching (e.g. ignore TTLs) for:
1. any record from a _num. zone (independent records
2. any record from *.num.net (hosted records)
3. any TXT record starting "@n=X" (all NUM records)
4. any TXT record over X bytes
Since DNS is becoming more and more consolidated with DoH and DoT, one decision from a player like Google or Cloudflare could have a big impact here.
I think it depends how the records are being used, if one domain has gigabytes of NUM records then I think resolvers are more likely to act against that domain in particular rather the protocol as a whole.
If gigabytes of NUM records were stored in cache from millions of different domains, then I'd hope that DNS resolvers would see the benefit of caching these records for users. If Cloudflare cached NUM records and Google didn't, then Cloudflare would be quicker for many operations (e.g. dialling a domain, fetching useful data with Siri / Alexa, etc).
1. any record from a _num. zone (independent records
2. any record from *.num.net (hosted records)
3. any TXT record starting "@n=X" (all NUM records)
4. any TXT record over X bytes
Since DNS is becoming more and more consolidated with DoH and DoT, one decision from a player like Google or Cloudflare could have a big impact here.
I think it depends how the records are being used, if one domain has gigabytes of NUM records then I think resolvers are more likely to act against that domain in particular rather the protocol as a whole.
If gigabytes of NUM records were stored in cache from millions of different domains, then I'd hope that DNS resolvers would see the benefit of caching these records for users. If Cloudflare cached NUM records and Google didn't, then Cloudflare would be quicker for many operations (e.g. dialling a domain, fetching useful data with Siri / Alexa, etc).