Yes, always. That’s what those who abuse DI and all other buzzwords keep missing; they are just tools and should be used sparingly because those in particular reduce a lot of code readability.
I don’t use those features extensively BUT on the rare occasion I want to go 5 layers deep into function definitions and have proper autocompletes and read docs right inside my editor, LSP has been absolutely indispensable. It’s IMO not about getting narcotics-level dependent on it, it’s about allowing it to empower you and save you time and brain cycles, allowing you to focus on your end task, not on 100% of the logistics.
I find the private / public visibility distinction overvalued. Testability > almost everything else, in my book. In Elixir we have boundary and with the tools I keep shilling for (especially ast-grep) you can write your own linter and integrate it in into a pre-commit GIT hook, CI/CD and whatever else you like.
Mind you, I still use defp and use it as much as I can get away with but the moment I feel that I have to test my stuff even more granularly, I don’t get a crisis of faith, I just replace the definition with def and sleep very well that night.