GitHub, GitLab, AirBnB, Twitch, Shopify, Fiverr, Twitter are written in Ruby on Rails. Of ours - InSales, UCHI.ru, Aviasales.
There are separate projects on the "rails" in almost any large company, for example, Google, Apple and Sberbank.
Is it promising to learn Ruby on Rails
Yes, if you like web development.
Ruby is in the top 20 most popular programming languages, and the number of jobs on Rails is comparable to the number of jobs on Django and Laravel.
At the same time, different people understand different things by “development”. For example, PHP is popular thanks to common CMSs like Wordpress, Joomla or Drupal. The number of vacancies on it is huge, but in fact they are not looking for developers, but webmasters to support the sites of small companies and finish the WordPress plugin. This is a boring and not very well paid job.
But most beginners, when they see the number of vacancies in PHP when choosing which programming language to learn, choose it. As a result, Ruby on Rails developers are always in short supply, so the average salary of a Ruby developer is higher (as well as Python developers).