Not a lawyer but in the scenario where proton closed the source but kept offering the build, even if gpl3 still applies since they’re the only copyright holder (no contributions) it’d only give them grounds to sue themselves?
From gnu.org:
The GNU licenses are copyright licenses; free licenses in general are based on copyright. In most countries only the copyright holders are legally empowered to act against violations.
In their defense, they also clearly label immich as under active development with frequent changes and bugs.
Edit: nvm I saw it was already discussed in another reply.