Starting with Rails LTS 2.3.18.19, it is possible to run Rails LTS with modern versions of RubyGems (2.6.13 at the time of writing) if you are using Rails 2.3 LTS with Bundler.
With RubyGems 2+, Rails LTS will restrict the following features which are now supplied by bundler:
config.gem ...
in environment.rb
.rails:freeze:gems
or rails:gems:unpack
.script/generate
).