User-Agent Reduction & User-Agent Client Hints
ImportantThis API is still being maintained by Google in Chrome, even after the deprecation announcement of most other Privacy Sandbox APIs.
Overview
User-Agent reduction is a mechanism implemented by several web browsers to minimize the amount of identifying information (used by fingerprinting techniques) shared through the User-Agent header and other APIs such as navigator.userAgent, navigator.appVersion, and navigator.platform.
User-Agent Client Hints allow web servers to explicitly request access to features (also designated as hints by the proposal) of the User-Agent header that are not exposed by default anymore. The idea is that browsers share a user-agent with low-entropy hints during the initial request, then servers that need them can explicitly ask compatible browsers for more hints through the proposed Accept-CH header. For a more detailed example, see this explanation.
Browsers would thus be technically able to mediate access to these user-agent hints of higher entropy. However, the proposal is unclear on how to perform so in practice, moreover, Google decided to not pursue their privacy budget proposal efforts that may have been part of a solution to this mediation problem.