It would be nice, but it's hard to know what probability is "too low". In short, the probability assigned by a model to a sequence of tokens can be arbitrarily low. There are things that are very unlikely to be said, but not impossible... and we still want them to be assignad some non-zero probability by a language model. So it's very difficult to choose a threshold that won't possibly exclude a large part of the sequences recognised by a language model.