I would say that 99.9% of horses competing at any level in Australia are microchipped, outside of rural “horse sports” type events. Anything attending pony club or an event on PC grounds is microchipped. Buy an OTTB? It comes microchipped. Any horse on the east coast vaccinated for HeV is chipped. Anything registered with a breed society? Chipped. To move a horse off my property to a club grounds requires a biosecurity declaration which requires a microchip number.
Good luck buying a horse - outside of a stationbred stock horse - in Aus that doesn’t come with a chip these days.
What you aren’t seeing much of is branding. The only horses I’ve owned with brands were two OTTBS and a stationbred in the 90s.
So this would be such a moot point here. Everything is chipped anyway! It would be like requiring all horses to have four legs prior to registration. Most eastern seaboard comps want your horses HeV vaccinated which is a yearly cost of $100+ a vet visit (no DIY permitted) plus registering a horse with EA is a ridiculous cost.
I think Eventing USA is simply adopting practices that are becoming standardised globally, not trying to nickel and dime grassroots out of existance.