Redhat (pre-RHEL) solved package signing around 1999 with RPM 2.0/3.0 by using PGP and later replacing it with GPG. Debian solved it around 2003 by also using GPG.

https://dan.drydog.com/rpm-signing-howto.html

https://www.cryptnet.net/fdp/crypto/strong_distro.html

With truly reproducible builds, it's possible to introduce distributed caching of artifacts and selective probabilistic rebuilds from source to attest/verify integrity in a distributed manner.