So what is the agreement percent needed for an upgrade or change? 88% orrr.
Discussion
Significant changes require social consensus. There is no way to ascertain the percentage of social consensus without some formal registry and vote system. Everyone takes a leap of faith in some regard, but ultimately chooses what makes economic sense to them. That may or may not be immediately consistent with their peers. Hence, Roger Ver forking off to Bitcoin Cash, and later Calvin Ayre and Craig Wright with Bitcoin SV.
The signaling performed for SegWit and TapRoot was miner signaling of "readiness", indicating that they could support an upgrade. There's no way to know if they were truthful or lying until such time came for the soft fork.
A soft fork works as an opt-in deployment by making rules more restrictive to those that follow them, and relies on an economic majority to avoid having their funds swept by those that don't upgrade. A hard fork on the other hand, is a change in subsidy, size, etc that REQUIRES people to change to that version or else they are cut off.
I do know for taproot to be implemented, they had to have a certain amount of the miners to agree.
That was the signalling for readiness following Speedy Trial. I was running the first taproot client for BIP 8, LOT=True which would have forced the issue at the same block height taproot was activated.
Nodes set consensus rules, but it helps to have miners enforce as well as it demonstrates broader consensus. If miners didn't signal, or gamed signaling as was done during segwit, then more would have switched to UASF approach.