Meh, I could be into it without them providing a FOSS implementation. Think of Microsoft's language server protocol. It's nice that VS Code is open source, but even if it weren't, we might still be using this protocol with rust-analyzer and neovim. Or any number of older protocols/formats with RFCs that didn't start with good FOSS implementations.
In the case of completion, if you can generate (less rich but still useful) bash/zsh/fish completion scripts from these files, program authors might be happy to use it even in the absence of a fancy terminal.
In the case of completion, if you can generate (less rich but still useful) bash/zsh/fish completion scripts from these files, program authors might be happy to use it even in the absence of a fancy terminal.