Arguably it's just the frontend. You can use old node in backend as much as you please. Frontend UI expectations evolve so quickly while APIs & backend can just stay the same, if it works it works.
It's not just "old node", it's also "old webpack" and "old vuejs" and "old everything". Yes, "it works" and strictly you don't really need to update it, but you're going to add a lot of friction down the line by never updating and at some point the situation will become untenable.
Old webpack and vuejs yes, because it's the UI which does get quickly old, but APIs or also things you would do with Go don't really have to change much as time goes on. A simple Express server and that's it.
This hasn't really been my experience, maybe if you're only using a single dependency (like express) it's easy but if you have any reasonable amount of npm deps it becomes quite untenable very very quickly. Mostly in ways you don't expect.
Like one dependency upgrading needing you to move to node v16 but doing so causes half your app to implode.
I don't really see this type of things in modern projects with other languages.
> You can use old node in backend as much as you please.
Not really, or at least not always.
I once tried to run some old Node project on Apple Silicon. It relied on some package that wanted to download binaries, but that ancient version didn't support Apple Silicon yet. Upgrading the package to an Apple Silicon supporting version required updating half of the other dependencies, and that broke everything. Eventually, I just gave up.