While I certainly think there could be a use case for memory, I'm not sure that either of those two questions fit it.
Rather than "forgetfulness" I would recommend we use forgetting. We want our tags to be non-judgmental, and I think that "forgetfulness" is a term that implies a repeated history of memory issues (often related to aging) while questions may be about a single failure of memory. This issue doesn't exist with forgetting.
... that being said, I don't think that How to remind an instructor about a message I sent them? is about memory or forgetting at all. Only the person the question is talking about can know if it was a lapse of memory or intentionally ignoring the OP, so the tag would be inappropriate.