Ruby applications without a `RUBY VERSION` in the Gemfile.lock may receive a default Ruby version
Change effective on 28 February 2024
Previously, it was possible to specify a full version of Ruby in the Gemfile even if it was not present in the Gemfile.lock. The Ruby directive in the Gemfile was parsed by bundler and emitted via the command bundle --platform ruby. This behavior has changed with bundler 2.4+, so only ruby versions listed in the RUBY VERSION key of the Gemfile.lock will be returned. If your application uses bundler 2.4+ and does not have a RUBY VERSION specified in the Gemfile.lock, it will receive a default version of Ruby.
It is strongly recommended that you have both a RUBY VERSION and BUNDLED WITH version listed in your Gemfile.lock. If you do not have those values, you can generate them and commit them to git:
$ bundle update --ruby
$ git add Gemfile.lock
$ git commit -m "Update Gemfile.lock"
Applications without these values specified in the Gemfile.lock may break unexpectedly when the defaults change.
If your app relies on specifying the ruby version in the Gemfile but not the Gemfile.lock and it is not yet using Bundler 2.4+, you may preserve this behavior by not upgrading the bundler version in your Gemfile.lock, however, this behavior is deprecated. It will be removed at a future date. It is recommended you lock your Ruby version now to avoid an unexpected breakage in the future. Ruby support article