[w3ctag/design-reviews] User-Agent Client Hints & UA Reduction (#640)

Hi there TAG,

I'm requesting a TAG review of User-Agent Client Hints and the plan to reduce the information available in the User-Agent header, and related JS APIs. The review in [issue #467](https://github.com/w3ctag/design-reviews/issues/467#issuecomment-634101437) was previously closed, but I’m requesting a new review now that Chrome is [interested in pursuing this plan again](https://groups.google.com/a/chromium.org/g/blink-dev/c/e3pZJu96g6c).

User-Agent Client Hints defines a set of [Client Hints](https://datatracker.ietf.org/doc/html/rfc8942) that aim to provide developers with the ability to perform agent-based content negotiation when necessary, while avoiding the historical baggage and passive fingerprinting surface exposed by the venerable User-Agent header.

  - Explainer: https://github.com/WICG/ua-client-hints#explainer-reducing-user-agent-granularity

  - Specification URL: https://wicg.github.io/ua-client-hints/

  - Tests: 
      https://github.com/web-platform-tests/wpt/tree/master/client-hints

      https://github.com/web-platform-tests/wpt/tree/master/ua-client-hints

  - Security and Privacy self-review:  https://github.com/WICG/ua-client-hints/blob/main/tag-security-privacy.md 
  - GitHub repo (if you prefer feedback filed there): https://github.com/WICG/ua-client-hints

  - Primary contacts (and their relationship to the specification):
      - Mike Taylor (@miketaylr - editor)
  - Organization(s)/project(s) driving the specification: Chromium
  - Key pieces of existing multi-stakeholder review or discussion of this specification:
Mozilla’s position: https://mozilla.github.io/standards-positions/#ua-client-hints

Early review of User-Agent information as Client Hints concept: https://github.com/w3ctag/design-reviews/issues/320#issuecomment-460926256 
Review of “Partial freezing of the UA string”, which was closed when our plans to proceed were paused due to the pandemic: https://github.com/w3ctag/design-reviews/issues/467#issuecomment-634101437 
  - External status/issue trackers for this specification (publicly visible, e.g. Chrome Status):
https://chromestatus.com/feature/5995832180473856

https://chromestatus.com/feature/5733498725859328 

Further details:

  - [x] I have reviewed the TAG's [Web Platform Design Principles](https://w3ctag.github.io/design-principles/)
  - The group where the work on this specification is currently being done: WICG
  - The group where standardization of this work is intended to be done (if current group is a community group or other incubation venue): W3C
  - Major unresolved issues with or opposition to this specification: There's some opposition recorded in the “Partial freezing the UA string” review
  - This work is being funded by: Google


We'd prefer the TAG provide feedback as (please delete all but the desired option):

  🐛 open issues in our GitHub repo for **each point of feedback**


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https://github.com/w3ctag/design-reviews/issues/640