Your question asks "where do I place a variable legend?" There is no one-size-fits-all answer to this, and your university (or advisor) may have guidelines for how theses should be organized. And, the decision will ultimately need to take into account the nature of your thesis and its organization. So in its current form, the best answer is "ask your advisor / university," hence the decision to close.
Now if you like, you can edit to clarify that your university does not specify anything and that you are looking for pros/cons and "best practices." That should salvage your question -- though, that might turn it into a duplicate of the post you linked, so you'd also need to explain why it's different.
The old post you linked I think is correctly left open. It asks "is it better to put the glossary at the beginning...or in the appendix," all other things being equal? This may seem like a subtle difference, but asking about the "best practice" or pros/cons of a particular approach is usually answerable.
That said, I do agree that the title question on the old post should probably be improved...perhaps I'll do so after this is resolved. As it is, the old post has generated a few correct but useless answers (e.g., "put it where it makes more sense to you"), which is something we try to avoid.