Probably reflects the ones the author has most used, I mean, I wouldn't start recommending stuff I've never touched either.

I imagine PRs could fill out more, like e.g. adding Haskell, Erlang etc etc.

This seems to be intended as a tree of achievements gained in the process of developing web, rather than a guide for what to use for any particular instance. Several of the paths I have happily skipped.

The backend could be further broken down into separate parts, each potentially using a different language, in order to give a clearer picture of the options and complexity. 1) web server (request/response), 2) search 3) database/store/access 4) data pipeline/ETL 5) payments

The intent seems to be "how to get a job in this area", not a comprehensive map of everything out there.

It's pretty messy, as it is.