I have followed the instructions here: https://openwrt.org/docs/guide-quick-start/verify_firmware_checksum?s[]=signature (with modifications for the 21.02 URLs), but of course it didn't verify because the GPG key listed there is for the 19.07 builds.
It would be very helpful if these instructions were updated for the 21.02 builds, or perhaps another page created just for the 21.02 builds.