The requirement for an EHSI comes from Section 7.1 of TGL-10, box 1.
The course selector of the deviation display shall be automatically slaved to the RNAV computed path.
EASA have accepted that for smaller Part 23 aircraft of relatively low speed (we have suggested max cruising below 150kts initially), that an auto-slewed H.S.I may not be required as the pilot will have time to adjust the course pointer manually at a leg change and re-position the aircraft to the new track without going outside PRNav limits.