Unfortunately with checkboxes, Webflow forms notifications do not have a way to exclude unchecked results.
There are hacks, like deleting those unchecked checkboxes just before the form is submitted, however that gets very messy with Webflow’s form data tracking. I don’t recommend it.
Two paths I can recommend…
One is to use a different form handler solution. If you submit e.g. to Zapier or Make, you can create your own email notification with some work.
A second approach is to replace those checkboxes with an option field, set to multiselect. It’s possible with some work to style the options with checkboxes, and to improve the multiselect user interfaces. I recall that MemberStack has a script to help with this.
I’d recommend a quick test with the multiselect option field first to ensure it’s giving you the form notification results you want.