Copyleft licenses allow unrestricted use and redistribution but require that any derivative works, if distributed, be released under the same license to preserve openness

Quotes that support claims

If content is published with a copyleft license (eg. CC-BY-SA, GPL), anyone can take it and use it and redistribute copies with no restrictions, but if you create and distribute a derivative work by modifying it or combining it with other work, the new work must also be released under the same license.

If you create and distribute a derivative work by modifying it or combining it with other work, the new work must also be released under the same license,” Buterin said, explaining slightly less permissive “copyleft” licenses like CC-BY-SA and GPL.