At the moment nerd-fonts seems to be based off of vorillaz/devicons. As it is what is linked everywhere on the website as well as here on GitHub. This repository is heavily outdated and unmaintained. The last commit was in 2017 and the last official release in 2015.
The devicons.ttf in this repo has also last seen a change in 2016.

Since then a lot of new projects came to life and new popular icons got added, that are not supported on there now. However there appears to be a fork of that repository, that actually still has regular updates. It also has about a third more GitHub stars in case that is any indicator of popularity.

Now this project may already somehow be used inside of nerdfonts. I can actually find the nf-dev-ruby_on_rails (e73b) icon on the cheatsheet which seems to be part of a commit from this April.

However if this already is the case at least the links and documentation in this project should be updated to reflect this. As it may be confusing for people (like me) to figure out where to go to propose an icon that they would want to see inside of nerdfonts.