I just put all the shit I don’t want to share on an entirely different computer and then use a kvm switch, to make sure I don’t accidentally share anything. Also old fart here.

As others have stated, the cleanest option for a single monitor setup is to either share a specific window, or start making use of multiple virtual desktops, sometimes referred to as workspaces. Windows, Mac, and Linux are all capable of it, now - the only difference is how you set up, arrange, and navigate them.

Linux options offer the most versatility, Mac's implementation is a decent balance between ease of use and scalability (with caveats), and the Windows native implementation is the newest entrant to this playing field... but it's an adequate offering that gets the job done for this use case.

I’m with you. A common example is the weekly meeting I chair. I can have everything we are going over on my left monitor. Center monitor has my meeting notes that I type up discussion points to complete after the meeting. Right monitor has slack up which is useful if I need to ping someone for input on something if they couldn’t be on the meeting or another resource. Even if I don’t get a response right away I typically have one by the time I’m finishing up the meeting notes. My mind works better having a separate physical monitor to break up my space. I have a 42” wide screen and 24” monitor for my personal computer and I want to downgrade the larger one. The 42” is too big for me, I enjoy snapping windows around with the arrow keys and changing virtual desktops feels too abrupt to avoid interrupting my mental processing. I think folks all process things a little bit different and I’m just happy I’ve found something that works for me and others have options that work for them.