Some users do not need the right to access the reports of the complete SiteID but only one or n segments.
The rights concept should be extended accordingly so that users only have access the segment data.
FYI this refs https://github.com/matomo-org/matomo/issues/1568
Adding the "New plugin" label since we likely won't implement this in core but someone could implement this using a plugin maybe.
Partially our recommended workaround be to track data into separate sites which lets you give access to different users and then get the aggregated view using Roll Up Reporting. Depending on the segment this might not always work but often.