It should be clear to discuss about crypto or bitcoin only.

With crypto, yes there are possible reversible transactions especially if the cryptocurrency you are talking about is a centralized cryptocurrency. Like ETH, Ripple, or any other same crypto projects.

With bitcoin, it is irreversible in theory and in reality it is almost irreversible. Reversibility can only be gotten if one pool has more than 51% of the total hashrates on the network, that is difficult. And the pool need to maintain such huge hashrate long enough. On bitcoin network, with long confirmed transactions, more than 30 confirmations, it is safe enough to say they are irreversible.