Why aren't tag edits and comments licensed as CC BY-SA 4.0 in post timelines?
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The CC BY-SA license only covers content. Tags are (apparently) considered metadata. In the original implementation, they triggered a license change, but after the bug report, they no longer do (and I agree with that, FWIW).
Comments are licensed under various CC BY-SA licenses, depending on when they were posted. At least SEDE and the API can tell you the license of a specific comment. You can also hover over the timestamp:
Including that information in the timeline might confuse users, thinking that the license of the comment would apply to the post as well. After all, it's the timeline of the post, and comments are second-class citizens, so they get less love than questions and answers.