The Chang Hon forms do specify targets (low/med/high section) for kicks, but the taegeuk and palgwae systems do not. And while not hitting the head high target zone will result in a lower score, it wouldn't prevent your from passing, assuming you performed the rest of the pattern to a reasonable standard. I don't know about other systems, but I suspect not. Perhaps
@Earl Weiss will chime in and tell us if not being able to kick 6' in the air would prevent you from earning rank in the ITF, but I can assure you that you can earn Dan ranks in KKW and MDK TKD without doing the splits.