Target display mode

Overnight my main computer (2019 iMac) updated itself (which it is set to do, but never normally does 🙄) to Sonoma.

With my configuration, the TDM still operates with the two 2012 TDs running Catalina. So in this case (at least) all is right with the world. Though it is not on Apple's radar, the TDM is one of the most appreciated functions offered by Apple, and is invaluable to both my research and aesthetic preferences; more screen realty (a bigger screen) is not better when you use computers for academic work (research, writing, teaching, online instruction, and academic publication), in which you need multiple screens to separate search browsers, .pdfs, e-mails, and Pages).

If the next OS fails to support TDM, I am not at all sure what I will have to do -- presumably, one would be forced to buy a Mac Mini and three Apple monitors (which are too big for my desk). I am not even sure if a Mac Mini would drive three Apple 4k monitors. In this respect, Apple's direction is less good.

The 2009-2010 iMacs did support connectivity to PCs

Only 27” 2009 & 2010 but that is ancient history. Target Display Mode was never something Apple wanted, that it worked at all was pure accident and certainly not worthy of discussion.