As originally envisaged in HTTP 1.0, HTTP 1.1 etc., the User-Agent header "consists of one or more product identifiers, each followed by zero or more comments which together identify the user agent software and its significant subproducts."

This open-ended mechanism allowed user agents to decide how much detail to reveal about subproducts. The example cited in the RFCs illustrates this usage, where the user agent choses to identify itself, its version and details of a constituent library:

User-Agent: CERN-LineMode/2.15 libwww/2.17b3

The UA-CH proposal by comparison is tightly prescriptive in the semantics of what user agents can chose to reveal about themselves. The Sec-CH-UA headers focus on a very narrow set of predefined fields leaving no natural place to express details such as constituent libraries, components and their versions.

This appears to result in a reduction of usefulness. The User-Agent header is currently used constructively to communicate limitations and capabilities of the user agents. A proposed replacement should not remove functionality that user agents chose to avail of.