G1 has read barriers for SATB marking. Whether a read barrier is used or not is a function of which GC is used so can’t really say “Hotspot doesn’t use read barriers”.
I don't immediately see why you'd need a read barrier for snapshot-at-the-beginning. You only need to trace references from gray objects to white objects that were deleted, which is a write.
However, ZGC will have read barriers and if Shenandoah ever gets integrated into Hotspot, it has them too.