I don’t think the “desktop metaphor” is particularly important or useful. It seems more like a gimmick to help people who’ve never used a computer before have something concrete to cling to. If you take away the desktop, tiling WMs are a natural place to end up. (Not that you can’t have a desktop with icons etc. in a tiling WM, but it loses a lot of its significance and isn’t really needed.)
Given that, the designer can decide if that’s an exception and everything else requires click-to-focus before being interactive, or to use focus-follows-mouse. People really seem to have individual preferences on this, so there’s no one right answer.
Newer versions are available in ports.
It's important to remember that fvwm is customisable so it doesn't need to look dated. Given enough time, it's possible to have an environment which suits the individual just fine.
In the wayland world, this is carried over to: https://cow-wm.codeberg.page/cow/