They do provide instructions for compiling from source, they just don’t support you at all afterwards. If you compile GrapheneOS and put it on your phone, they say “you are not running GrapheneOS” at that point. Unlike Debian or Ubuntu, where every package can be replaced by a hand-compiled version, and it’s still Debian or Ubuntu.
Totally, since being able to compile from source is very much a security issue.
They do provide instructions for compiling from source, they just don’t support you at all afterwards. If you compile GrapheneOS and put it on your phone, they say “you are not running GrapheneOS” at that point. Unlike Debian or Ubuntu, where every package can be replaced by a hand-compiled version, and it’s still Debian or Ubuntu.
You can compile from source