In this video, Richard Stallman claimed that Linus Torvalds created Linux kernel as a proprietary software and then liberated it ('liberated' is the term Stallman used).
Assuming so, when was the Linux kernel made libre by Linus Torvalds?
The Linux article on English Wikipedia says as of now:
Torvalds initiated a switch from his original license, which prohibited commercial redistribution, to the GNU GPL.
but the source doesn't mention a specific year like 1992.