During 2017, miner-signalling for activation was used as a political tool to hostage/stop the network from doing an upgrade that the community wanted. BIP8/UASF is a method of activation wherein the full nodes decide, not the miners. The miners simply follow.
Does activation just mean a node is running 0.21.1 or do they have to take extra steps after the the upgrade to 0.21.1?
Put simply...
Either:
- Miners activate the fork. This happens when a certain % of miners are making blocks with a special "taproot ready" flag embedded in their mined blocks. This % of blocks must continue for ~2 weeks (for 2016 blocks)
- Users activate the fork. This happens when the timeframe for activating the fork expires
It's a little more complicated than though. Not much.
As of yet, the % for triggering the 2016 block lock-in stage hasn't been decided. Suggested figures are 90% or 95% of blocks (95% was used for some recent fork activations). The overall timeframe is also not exactly decided, but 1 year (measured in blocks...) seems to be a popular suggestion.
It is trivial to create an unlimited number of full nodes, and the cost of doing so is near zero. It is not trivial to purchase and operate a miner, and the cost of mining is far from zero. The miners have a real economic incentive to do what is best for the long term interests of bitcoin.
The above does not mean that miners should have absolute veto power over changes to consensus rules, but it does mean that miners should first be consulted before changes to consensus rules are made. It will always be best to get the miners to agree to changes to consensus rules, and if they do not initially agree, substantial effort should be put into trying to convenience the miners that said changes are in the best long term interest of bitcoin (and the miners' long term business interests). Rejection of changes to consensus rules by the miners should be given significant consideration and the community should consider if it is best to move forward with changing consensus rules without the consent of the miners.
To put the above a different way, a BIP should be first attempted to be implemented via gaining the consent of the miners. If the miners do not agree to implement a BIP, a USAF can be considered, but significant weight should be placed on the fact that the miners do not believe the BIP is in the long term best interests of bitcoin. A USAF implementation of a BIP will be contentious, and it is always better to implement a BIP via non-contentious means.