In the case of the word undeletable, none of the main English dictionaries seem to have an entry for it. (Or at least for the adjective.)
But this is what's provided by Your Dictionary:
(computing) That cannot be deleted; indelible.
(computing) That can be undeleted.
Like any other use of English, ambiguity needs to be resolved through context or a rephrasing of the sentence.
Nobody can remove that file; it is undeletable.
Here, the only logical interpretation is "cannot be deleted."
Thankfully, if you delete that file, it is undeletable.
Here, the only logical interpretation is "can be undeleted."
It is undeleteable.
This sentence (which I'll assume is ambiguous) would need to be rephrased in one of two ways.
It cannot be deleted.
If deleted, it can be recovered.
In other words, don't use the word if using it would result in confusion.