The reason lies in the SSE3 instruction set.
Google is preparing to include support for the SSE3 instruction set in a list of minimum system requirements for the Chrome browser. When this happens, users who use PCs that were released before 2005 will face a problem – they will no longer be able to work with the browser since CPUs older than Intel Core 2 Duo and AMD Athlon 64 lack support for the SSE3 instruction set.
In the future, Google wants to use more modern instruction sets in the Chrome tandem (specifically SSSE3; not to be confused with SSE3), and so they are now forced to raise the instruction set requirements to SSE3.
Google conducted research and found that the number of Chrome users using non-SSE3 processors is small enough to raise the minimum system requirements.
In general, this step is not something out of the ordinary. What is more surprising is that such systems still work. Perhaps no one, apart from a handful of enthusiasts from the retro community, will notice anything. For the latter, such problems are no longer news: modern software requires modern hardware, which is not surprising.
It’s unclear if this change will affect other Chromium-based browsers like Microsoft Edge. But for the owners of ancient PCs, it is not lost since Mozilla Firefox does not plan to include SSE3 in the list of essential requirements yet, so at least one option remains.