Chrome Phasing out Support for User Agent [infoq.com]
Google announced its decision to drop support for the User-Agent string in its Chrome browser. Instead, Chrome will offer a new API called Client Hints that will give the user greater control over which information is shared with websites.
The User-Agent string can be traced back to Mosaic, [wikipedia.org] a popular browser in the early '90s where the browser simply sent a simple string containing the browser name and its version. The string looked something like Mosaic/0.9 and saw little use.
When Netscape came out a few years later, it adopted the User-Agent string and added additional details such as the operating system, language, etc. These details helped websites to deliver the right content for the user, though in reality, the primary use case for the User-Agent string became browser sniffing.
[ . . . . ] Browser sniffing continued to play a significant part in determining the browser capabilities for many years, which led to an unfortunate side effect where smaller browser vendors had to mimic popular User-Agents to display the correct website - as many companies only supported the major User-Agent types.
As a result, the most significant usage for the User-Agent remained within the advertising industry, where companies used it to 'fingerprint' users, a practice that many privacy advocates found to be problematic - mainly as most users had limited options to disable/mask those details.
If advertisers (other than Google) are unable to fingerprint our browsers we might be condemned to having fewer ads on our web pages to watch.