Linux 5.13 was the first kernel release with Landlock incorporated, but the Landlock project is from 2016 also.

I found the announcement email for Landlock posted to the lkml[1] where the author compares the project to Pledge. There's also his talk[2] from 2016 if you're interested. I was certain landlock predated pledge, as I thought the website or earliest talk was from late 2015, but I am less certain now, indeed I seem to have been wrong in my claim.

As for either being the first, at the very least Seatbelt from Apple has a paper dated 2011[3] and was released with macOS 10.5.

[1] https://lwn.net/Articles/700607/

[2] https://archives.kernel-recipes.org/document/landlock-lsm-un...

[3] https://www.ise.io/wp-content/uploads/2017/07/apple-sandbox....