Codidact seems like a good alternative. Some good things:

• Similar site organization of communities
• Same system of upvotes/downvotes
• Cleaner site design than Stack Overflow imho that stays consistent even without Javascript
• Core components licensed under AGPL-3.0, rest (scripts, styles, etc.) under MIT: https://github.com/codidact
• Posts by users of the site are explicitly licensed under a license.
• Is active, although of course not as big as stackoverflow.com