To answer you question Home owners are not allowed to advertise for a sitter when they have a banned breed of dog.
It’s against THS T&Cs
5.2.11. not have any inherently dangerous pets (such as venomous snakes or constrictors, primates, wolves or wolf hybrids, non-domesticated cats, alligators), banned dog breeds, or any animal with a history of attacks on pets or people;
But it depends which country the sit is in and whether the breed is banned in that country. The U.K. ban on xl bully’s has been discussed here :
If the dog is banned in the country the sit is in then this is a breach of the T&Cs and you can reported this to member services e-mail
Support@trustedhousesitters.com