Answer a question

After you join an Organization on Github you automatically subscribed to all of its repos and you start getting all those updates you don't really want. To unsubscribe you need to go per repo and do it manually. Is it possible to unsubscribe from all repos within a particular Organisation at once?

Answers

It should be possible since Jan. 2022, 6 years later.

Unwatch all repositories owned by a given user or organization

Users can now unsubscribe from all repositories owned by a given user or organization.

Navigate to github.com/watching to find a list of the repositories that you are subscribed to.
The Unwatch All button gives you the option to unsubscribe from all repositories, or just the ones that belong to a specific user or owner.
It will appear when you are watching all activity or custom notifications on over 10 repositories.

Clicking any of the dropdown options opens a modal to confirm unwatching repositories owned by the selected user/organization (or all repositories).

You can read more about how to unwatch a single or multiple repositories in the 'Managing your subscriptions' documentation