When a user decides to delete their own account or when for some reason the account is deleted, then you can't see any information about the user, you can't click on the id "name" of the user.
Is it possible to know what were all of his/her questions and answers?
Notice that this question is different from this other question
An example to explain this was provided by Martin Sleziak (in the comments section)
These are two different questions. For example, if I delete my account, my posts will show user8297 as the poster. This question is asking whether from knowing the deleted users' id (8297 in this example), it is possible to find all posts by that user. The question which was suggested as a duplicate is asking whether from id we can find out the username which the user was using before the deletion (in this example Martin Sleziak).