Will a ledge of drywall get deformed with time?
Oh it will sag so bad!
Use piece of MDO or birch plwood with a glued on or iron on edge, if you need to keep the thin edge. If the run is over 8 ft. glue the but joints with a biscuits or spline. 3/4" plywood will work best for this and you can profile the glued on edge in this case to a smaller profile if you want it thinner than 3/4"

If it is a matter of preferring it to be in the beam, but do not want to ruin its integrity, you can box it in with drywall, add a nailer on the ceiling and create a wider box to hold the light beside the beam, but contained in an enclosure. If the beam is structural and esthetic, you may be able to re-wrap the beam in the same wood and finish, just make it wider.