Legacy Transaction

Hi, simple question: are legacy transaction (before EIP-1559) still valid. They don't consider the gas fee to be split between the basic and priority fee but I read that they could still be valid. I tested this transaction type in a testnet (rinkeby) and it worked but I'm not sure it was because of the low gas price in there (that probably didn't require a priority fee to be sent).

If anyone has a link or reference about this please link it.

Thanks