There is nothing to stop people registering as whatever gender (it’s listed as gender in Park Run) they want…although I think it’s only two in England.
it would annoy me, I am a runner, a plodder but at Park Run I tend to do okay for my age group and as a woman. Compared to men my own age of a similar ability, I am rubbish. When I have been a volunteer at an event which averages 150 runners per week, the top 20 runners are male, then a couple of young woman, then loads more men then into the mixed plodders.
Park Run is wonderfully inclusive, there are walkers, trip hazard toddlers, dogs and lots of runners obsessed with PB who use Park Run as a warm-up.
It will mess up the stats if it becomes widespread but then most runners use their PB to monitor progress. It would really upset me if this became widespread in paid for events, as it would just be depressing to be wiped out by a male bodied person the same age but probably not as fit. I enjoyed a women only event a few years back, mainly as I finished higher even through the elite runners completed it in half my time. It was the best atmosphere at any running event, lack of men and small children helped.
I think the best solution would be to record gender id along with sex but it not compulsory to complete. Although regulars would soon know who was trans and a regular runner then probably recalculate the stats themselves for own use.