The tag wiki for star-wars-legends gives the following usage guidelines:
Use this tag if your question references Legends sources and/or to indicate that answers are permitted to use Legends sources.
(Note that I'm the user who added those usage guidelines to the wiki. I proposed/created the tag as well.)
We should follow the usage guidelines for all questions, regardless of when they were asked. However, it may be difficult to determine the asker's intent to include or exclude Legends for the question. Just because the question was asked before the introduction of the Legends brand doesn't necessarily mean the asker was looking for a Legends answer -- so we shouldn't add the tag to all questions asked before the introduction of the Legends brand. The tag should only be added if there is clear evidence that the asker intended to include Legends in the question or if the asker was willing to accept answers that use Legends source(s). We can safely add the tag to questions that exhibit one or more of the following:
- The question refers to a Legends-only event, character, etc. (e.g. a question about Jaina Solo is clearly a Legends question since that character only appears in Legends and is unlikely to ever be added to canon).
- The accepted answer (if there is one) uses only Legends sources. Clearly the asker permitted Legends sources if he accepted an answer that only uses Legends sources.
- The question body explicitly indicates that Legends/EU sources are permitted. For example, in this question of mine I say that Legends sources are permitted as long as they do not contradict canon.
- The asker indicates in a comment that answers are allowed to draw from Legends sources.
Unfortunately, these criteria are too complicated for an automated script so we'll have to add the tag to the appropriate questions manually. Feel free to do so, but avoid editing too many questions at once (the edited questions should not take up more than about 5 of the top 15 questions on the front page).