While true, many of those OSes are not updated anymore. So good old Vim (4, 5, 6, 7) will serve them just fine. More than that, several of them (Amiga, BeOS, etc.) run on underpowered machines so the latest and greatest Vim updates won't run on them anyway. We could say that they're well served by Vim 5, for example.
In your concrete example of a feature being removed, Neovim is actually removing a misfeature: the interactive ex mode, not the command line ex mode. The feature they're removing is akin to: http://xkcd.com/1172/.
I use ex maybe once a year, when $TERM is screwed up or latency is terrible, but to me removing it is a red flag that says the NeoVim folks don't really grok vi.
This was my initial take, but all that's really being removed is the ability to switch between ex mode and visual mode - to which my objection is substantially weaker (to the point that code clarity and separation of concerns are probably more important).
In your concrete example of a feature being removed, Neovim is actually removing a misfeature: the interactive ex mode, not the command line ex mode. The feature they're removing is akin to: http://xkcd.com/1172/.