Disable Theme and Plugin Auto-Update Emails

You can simply enable this to disable these notifications. Deactivating the plugin then has it resume default behavior. A simple implementation that's quick & easy to utilize. Consider enabling/disabling the plugin the setting for whether or not these email notifications are disabled or not. Also, this does let notifications about failed updates to be delivered by default as they can be important. There is a "Disable failed update email notifications" setting on Settings => General to disable update failure notifications as well, if/when there's a need for it.

By default, notifications about failed updates will still be delivered, but there is a "Disable failed update email notifications" setting on Settings => General to disable update failure notifications as well, if/when there's a need for it.

Try out Disable WordPress Core Update Email (or similar) as that tackles that separate group of emails in this same lightweight fashion (use official WordPress filters to simply disable the email[s] as described.) These two plugins can work together to remove all these notifications (hence why this doesn't also remove core update notifications). Meanwhile, one can use one or the other if a specific set of emails is desired to be blocked (ex. I personally like the core update notifications since they're not too frequent while potentially being substantial so I keep those enabled while I then just use this plugin to disable the more frequent & typically less impactful plugin and theme update notifications.)