As of 10.2, Citus columnar allows you to create indexes and do plain index-scans on them when it's beneficial, but does not yet support index-only and bitmap scans.
So the real position is: plain index scans work, index-only and bitmap scans don't. Microsoft's Citus 14 page appears to be stale: its wording, "No index scans or bitmap index scans", reads as if it predates the 10.2 work.
https://learn.microsoft.com/en-us/postgresql/citus/table-management?view=citus-14#limitations
As of 10.2, Citus columnar allows you to create indexes and do plain index-scans on them when it's beneficial, but does not yet support index-only and bitmap scans.
So the real position is: plain index scans work, index-only and bitmap scans don't. Microsoft's Citus 14 page appears to be stale: its wording, "No index scans or bitmap index scans", reads as if it predates the 10.2 work.
https://learn.microsoft.com/en-us/postgresql/citus/table-management?view=citus-14#limitations