Controlling Subscriber Notifications: Solution
In the end we found a pretty simple solution to our problem: how to keep notifications from going out to subscribers using Jetpack in Gutenberg.
We went into Jetpack > settings > discussion and turned off the option to let people subscribe (“Let visitors subscribe to new posts and comments via email”). Then we published the post we didn’t want notifications for before we turned it back on. It doesn’t send out notifications for posts published while it is turned off, and so this is what we’ll be doing for now!