"New rules activated" warning after upgrading to 0.20.0 I've upgraded a node from the previous release. It's up and running but for some reasons I get this warning.
How can it be related to the upgrade?

$ bitcoin-cli -testnet getblockchaininfo | jq
{
  "chain": "test",
  "blocks": 1764182,
  "headers": 1764182,
  "bestblockhash": "0000000000004a0e9cc1fbc2a21938feef1a26cd480483b3c498ae2ef16b46f8",
  "difficulty": 65536,
  "mediantime": 1591710673,
  "verificationprogress": 0.9999998260147338,
  "initialblockdownload": false,
  "chainwork": "00000000000000000000000000000000000000000000015bbad4a75d4239391e",
  "size_on_disk": 27411142847,
  "pruned": false,
  "softforks": {
    "bip34": {
      "type": "buried",
      "active": true,
      "height": 21111
    },
    "bip66": {
      "type": "buried",
      "active": true,
      "height": 330776
    },
    "bip65": {
      "type": "buried",
      "active": true,
      "height": 581885
    },
    "csv": {
      "type": "buried",
      "active": true,
      "height": 770112
    },
    "segwit": {
      "type": "buried",
      "active": true,
      "height": 834624
    }
  },
  "warnings": "Warning: unknown new rules activated (versionbit 28)"
}
Miners have been using the version field of the blocks' headers as an "extra nonce" (*).

However, this version field has been used to signal readiness from miners to deploy a soft fork (update to new backward-compatible rules).

The version field being used to synchronize the update to new consensus rules is the reason you see the warning.

The version field value not being enforced by the consensus, and the development of new techniques that increase miners' revenue is the reason they can "disgress" the protocol. 

(*) In this specific case, it's known as version rolling and is likely part of using overt asicboost.



EDIT - Some reference of discussions about how to deal with this in bitcoin-core :


BIP320 implementation
Discussions on an issue
GUI warning removal