Light on Nostr: But for local software, regardless of whether the source code is available or not, ...

But for local software, regardless of whether the source code is available or not, security researchers can ensure that there are no backdoors taking control away from the user. And regardless of whether forks are allowed, they are still possible for people who are willing to break the law (albeit if the software is being shared in a lawless zone, malware is an issue and revealing of personal info is a risk).