Should be able to require a checkbox field via editor view InspectorControls toggle.
The field itself still supports required adding the "required" attribute to the code view like this:
<!-- wp:jetpack/field-checkbox {"label":"Checkbox Test", "required":true} /-->
But this is not user-friendly. A typical user would expect the ability to add this attribute using a GUI toggle.
Removed in #15876.
No InspectorControls "required" toggle exists.
Screenshots

You should be able to control this setting here:

cc @apeatling Maybe we should add a toggle in the inspector controls as well?
Adding the "Needs Design" label as well to get another opinion.
This is missing on a single checkbox field. I will work on a PR to add this. I mistakenly thought required didn't apply to a single checkbox because it would always force it to be checked regardless of value.
@jeherve Added a PR to fix this here https://github.com/Automattic/jetpack/pull/16162
@apeatling we've had a question about this in the wpcom forums as well. Will this fix be applied to wpcom automatically with the JP release that contains it?
Looks like there's a Calypso issue for this as well in https://github.com/Automattic/wp-calypso/issues/42956
Will this fix be applied to wpcom automatically with the JP release that contains it?
I've just applied the fix to WordPress.com; the problem is fixed there, and will be available for Jetpack and Atomic users on July 7.
Thanks, @jeherve
Most helpful comment
This is missing on a single checkbox field. I will work on a PR to add this. I mistakenly thought required didn't apply to a single checkbox because it would always force it to be checked regardless of value.