You don't need credit to have a contactless debit card, they're just locked down to require online transactions so that it's impossible to go overdrawn. In normal operation a TfL gate is online (ie able to talk to your card issuer) so it can work with any type of card.
Sure, but many basic bank accounts (those available to people with extremely poor credit history and no income) don't have a contactless debit card anyway, such as the one Bank of Scotland does: https://www.bankofscotland.co.uk/bankaccounts/compare-curren...
TfL gates aren't online in anything but the most basic way - the first time you use a contactless card on TfL each day they'll place a hold for 10p to verify the card is valid. They then modify that to request the total cost of travel for the previous day after that.
