Vitalik Buterin has shifted from favoring permissive licenses to supporting copyleft licenses like GPL and CC-BY-SA, which he sees as legal safeguards for openness by requiring shared source code in derivative works

Quotes that support claims

He noted, “Historically, I was a fan of the permissive approach (eg. my blog is under the WTFPL). More recently, I am warming up to the copyleft approach.

In contrast, copyleft licenses like GPL or CC-BY-SA require derivative works to be shared under the same terms, including the publication of source code, creating a legal safeguard for openness.