A reverse withdrawal is when a player cancels a withdrawal that is still pending and returns the money to their gambling balance. Many sites used to allow it, sometimes holding withdrawals for days, which made it easy to gamble winnings away before they arrived.
The Gambling Commission banned GB-licensed operators from offering reverse withdrawals from 31 October 2021, at the same time as the online slot game design rules. Once you request a withdrawal, it goes through.
If you want to keep a win, withdrawing it promptly and setting a deposit limit are both sensible steps.