My understanding is that the PSL is good-enough and avoids somewhat costly/unreliable TXT lookups for every domain when only a very tiny fraction of domains wou

My understanding is that the PSL is good-enough and avoids somewhat costly/unreliable TXT lookups for every domain when only a very tiny fraction of domains would actually want this treatment. There is also a bit of security risk since browsers use this list to set cookie restrictions. If it were in DNS, which the vast majority of people use unencrypted, an adversary could manipulate responses to either (a) drop the TXT record altogether so the domain is not restricted or (b) craft a response in which the domain disables the behavior.