I would say the same as Jack, that if you primarily kneel, you don’t really need the footbrace, but it might still be nice to have it for those times when you are not kneeling. Almost everyone who kneels in a canoe takes occasional sitting breaks to get their legs feeling limber again, and if your seat has a forward slope to it (most kneelers set up their seat that way), the footbrace makes that forward slope work really well for the plain old sitting position. In fact, while sitting on a sloped seat with your feet against a brace, you are “locked in” far better than what’s possible with a footbrace and a normal flat seat (I’m talking about bench seats here, not tractor seats). In the end, your boat will be more versatile with a sloped seat and a footbrace.
I learned this by accident after I started out solo paddling while sitting on a flat bench seat. I realized I really needed a footbrace in that situation, and after a year or so when I switched to kneeling and put a slope on the seat, viola, sitting during my occasional breaks away from kneeling put me in a situation that was far more comfortable and secure than it had ever been when sitting on a seat that was flat - all thanks to the footbrace. I sold that first solo canoe and as I got a few others solo boats, I set them up for kneeling and the only reason I didn’t add footbraces to any of them was because, as another poster has mentioned, it would have been an extra thing to futz with, but I do think a footbrace is handy.