I think that this checkbox has more to do with making the segment available to other products in the Experience Cloud (e.g. Target), and less to do with the segment being made available in other report suites. To share segments to a wider audience, you'll need to go to Components > Segments, select one or more segments via the checkbox, click the "Share" icon at the top of the page, and then choose the individuals/groups/all option.
I did not receive an email about a 75 segment limit, and don't see it in the Release Notes.