Not a daft question - I honestly dont know.
The tannin will be there tho & any pony/horse that is an avid acorn scoffer will often still gorge on the older brown acorns.
I had a small pony down with colic one January 20+ yrs ago when a livery (I dont have them now!) let their pony into a fresh paddock when I was out for the day. Pony went straight over to the oaks & proceeded to spend probably the whole morning gorging on old brown coronet-deep acorns.
Had vet out but the impaction was so bad that nothing could be done without vast expense.
I'm pretty careful still these days & make sure that the majority are cleared in paddocks. The dieters are not allowed in any paddock till all acorns are cleared.
Perhaps a vet/poisons expert might be along soon, but from the stuffing/colic side - I'd not turn out with a mass of acorns in any field
As my neighbour has lost six sheep to acorn poisoning this autumn, I am not risking it! We had a horse some years ago who ate some which were brown and looked a bit like pony cubes, the toxic effect was not something I would want to see ever again.
we used to be told that once there had been a few hard frosts and the acorns go black and start to rot, they are ok-although I thing its more likely that animals won't eat them then. Wouldn't take any chances personally, its a bad year for them.