I do this on Hyprland all the time, but it’s a tiling window manager. I’m not sure any desktop environments have support for it.

In the upcoming Cosmic desktop by Pop!_OS, workspaces can be monitor specific, just like MacOS

No, it can't be done that on kde or gnome. Here are two such questions on the "other" site, as some people have offered some workarounds there in the comments: https://www.reddit.com/r/kde/comments/bxrrn3/is_there_any_way_to_separate_kde/ and https://www.reddit.com/r/kde/comments/lijbee/multi_monitor_independent_desktop_workspace/

Gnome lets you do this on the primary display, but afaik it's not possible on the secondary/tertiary displays.

This is a very old feature request for KDE. It has not been implemented in almost 20 years so its probably very hard to do with kwin. Neal Gompa thinks this wayland protocol could make it possible in future, but with the current speed wayland protocols are accepted, this could take another 20 years.

(on the Linux side, I »think« only tiling window managers have really experimented with and taking advantage of this sort of thing)

This is unfortunately stupidly difficult on most Linux desktops, because the EWMH standard doesn't support it. You mostly find support for it in niche desktops, like various tiling WMs or the Enlightenment DE.

You can have multiple workspaces per monitor on GNOME, just check your multitasking settings. I do this for my workflow since it's more efficient for me.

When browsing for workspace indicator extensions the other day, I alao saw this: Gnome extension Switch workspace on active monitor

It wasn't useful to me, because I use only one huge monitor, but might be what you're looking for. it was easy to go back and find it, because I remembered my search term was just "workspace".

I don't think you can do it with kwin (KDE wm), but if you're using xorg, you can replace it with something like i3wm. Do note you'll lose the desktop, and will need to unbind most KDE shortcuts. It's definitely worth it though.