Sonos BOOST Wireless Range Extender is no longer available through SONOS but can still be purchased through a third party (Argos, for example). Does anyone know why?

Sonos appears to be retiring the product, as they are essentially unnecessary. You can get the same effect by wiring a regular Sonos device to your router.
 

Also, home routers are much more capable now than they were when this product was released, and consequently much more capable of handling a music stream without challenge. 

Note, however, that end of manufacture and sales of a Sonos product by no means indicates that it is no longer supported. Sonos has, in the past, indicated at a minimum, a five year support window past manufacturing / sales end. That being said, however, none of the last batch of Sonos product can connect to the SonosNet signal created by the BOOST. However, it’s not an issue, I have the majority of my system currently connected to a BOOST, and the remainder, the newer speakers I have, hanging off my WiFi router’s signal, they all work perfectly well together. 

So, for example, could I wire my Beam to my router? I’m using the extender to help with other devices in our home, would the Moves, Roams and Amp now ignore the extender and communicate directly with the Beam?

Yes, that would work. Just make sure the WiFi/radio on the Beam is not turned off. 

Except for the Move and Roams: portable Sonos devices cannot participate in SonosNet, they will have to use your WiFi.

Ah, great point, I mentally skipped that in the post. Moves and  Roams have never been able to connect to a SonosNet signal, they can only connect to a regular WiFi signal. 

The Beam is using fairly old innards, wiring a newer Sonos device is likely to get you a better connection. If you see one a Bridge is the previous version of a Boost and is so old it should be avoided.

A lot of Boost owners have pulled theirs and stuck it in a spares bin as they see a better network without it plugged in.

Should we add the Era line to the list of won’t do SonosNet Sonos?

https://support.sonos.com/en-us/article/choose-between-a-wireless-and-wired-sonos-setup

It all seems to be heading in the direction of running a Sonos system on a WiFi mesh home network, of that I’m in no doubt. We are seeing support for WiFi-5/6AX at the moment, but no doubt that will lead onto WiFi 6E and WiFi 7 in the not too distant future, I suspect. I think SonosNet will eventually fade from view, but may just lurk in the background as the transition continues.

Hopefully, it remains functional for those devices that currently can connect to it. I have no plans of retiring my BOOST, even if some of my speakers need to hang off my WiFi.