I would be interested in licensing all of my posts, and comments (or any other applicable user-created content) under CC BY-SA 4.0. Is this legally feesible? How would one go about this properly? Is it enough to just state in a bio something like "All of this users posts are licensed under CC BY-SA 4.0", or would I also have to add "This work is licensed under CC BY-SA 4.0" to the end of every post, and comment that I make?
It sorta depends on what you hope to achieve. It wouldn't be profitable to rely on the licensing in hopes of getting money from news companies or publishers, since you're unlikely to have unique, viral info.
As far as training material for ai, it's really not worth it. Legal precedent isn't established enough. They scrape fully copyrighted books and news sites, and we don't even know if those lawsuits are going anywhere. Other legal precedent for scraping for use in any data analysis or academic use, or even for commercial data analysis, pretty much allows us to scrape anything that's public-facing.