AFAIK you need to pay at least 1 sat/vb to be relayed and get mined by normal mempool policy. There's nothing in consensus code afaik preventing either mempools and miners from facilitating 0 sat/vb transactions - however it is a DOS vector.

More here: https://bitcoin.stackexchange.com/questions/112531/how-to-find-nodes-forwarding-transactions-with-less-than-minrelaytxfee