The version of ESLint you are using.

7.31.0

The problem you want to solve.

Support Ergonomic brand checks for Private Fields (#x in obj syntax) officially, as it has reached stage-4.
refs:
https://github.com/tc39/proposals/blob/master/finished-proposals.md
https://github.com/tc39/proposal-private-fields-in-in

Your take on the correct solution to problem.

• Ecma262 update: Normative: add #x in obj syntax tc39/ecma262#2290
• ESTree update: add privateIn to es2022 estree/estree#252
  Maybe we just merge https://github.com/estree/estree/blob/master/stage3/private-fields-in-in.md into es2022.md.
• Acorn update: Support ES2022 class-private-fields-in syntax acornjs/acorn#1068
• eslint-visitor-keys update:
  Maybe we have no task on it.
• espree update: feat: support for private-in syntax js#521
• eslint-scope update:
  Maybe we have no task on it.
• eslint update:

Are you willing to submit a pull request to implement this change?

It's difficult for me to change the parser.
I may be able to do other necessary work.