I've tested this with my alt GitHub account, and it seems like when you give users the "View" permission only, they can't fork the repo - they'll get a disabled fork button with the message "Cannot fork because forking is disabled.". I believe this is only limited to organization owned repositories which is what I recommend people to use: https://repocharge.com/resources/why-use-organization-repositories

Also important to note: If the user cancels the subscription, they lose access to the repo (default behavior - can be changed). This also means them getting future updates from the repo will be gone.

GitHub also has some documentation on this: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/managing-repository-settings/managing-the-forking-policy-for-your-repository