This sounds similar to SLIME (and its fork SLY) for Common Lisp. These tools use their own protocols rather than LSP in part because they pre-date LSP but also because LSP is not a good match for everything they do. In addition to the usual LSP functionality like symbol lookup, xref and completion they also provide a very good Lisp REPL. It is easy to have the server side (called swank) running as part of an existing service which sounds a lot like what the post describes. For example I’ve used SLIME to connect to my live StumpWM sessions.

I thought that slock was too complicated so I wrote a tiny one for myself in Go using xgb. Less than 100 lines and pretty straightforward but it makes some assumption about my personal setup so not public.

Not worried at all, I’ve moved on many years ago.

EXWM. I am a longtime Emacs user so merging the concepts of Emacs buffers and X windows is a huge benefit. Only one set of keybindings to worry about, all of my Emacs window management stuff works for X windows too. One less external dependency to worry about too. In a new environment (like when starting a new job etc) as long as I have my Emacs config I am good to go.