I don't think the onus is on the original project to maintain compatibility with a fork. Neovim made it pretty clear from the beginning that they were going to fracture the ecosystem and now we have colorschemes in Lua that can't be used in vim... say what you will about Emacs, but at least all of their distros can run the same code.
From my perspective as a vim user, neovim has only made my life worse by splitting plugin authors into two camps without any real benefit over what we had in vim. The only good thing about neovim is it caused some nice features to be added to vim, which the neovim authors could have just contributed themselves without trying to fight for control of the ecosystem with Bram. Neovim has really just made things worse for everyone.
From my perspective as a vim user, neovim has only made my life worse by splitting plugin authors into two camps without any real benefit over what we had in vim. The only good thing about neovim is it caused some nice features to be added to vim, which the neovim authors could have just contributed themselves without trying to fight for control of the ecosystem with Bram. Neovim has really just made things worse for everyone.