I think validity in general is a somewhat ambiguous term. On one hand, a node verifies a transaction using a list of criteria. If all the criteria are met, the transaction is considered valid. One such criterion is: “For each input, the referenced output must exist and cannot already be spent” (Antonopoulos, Mastering Bitcoin, p. 235). So if all the inputs of the new transactions are UTXOs the criterion is checked.
At the same time, when discussing the “51% attack,” Antonopoulos notes that “The more confirmations elapse, the harder it becomes to invalidate a transaction with a 51% attack.” This implies that a transaction can be invalidated through a chain reorganization.
So, from a miner’s point of view, the validity of a transaction is a binary property. However, from the broader network’s perspective, it is probabilistic.
