As per a request by @headius, I am posting here the issue I reported on the JRuby Matrix chat :

Hi,

I just wanted to note that I stumbled on an issue that happens when upgrading Bundler to 2.6.x in JRuby. It fails on require 'bundler/setup' because Gem::Specification#ignored? doesn't exist. To fix the issue, I had to upgrade my gem system version to 3.6.3 (from 3.3.26). I might end up having to lock Bundler to an older version in my project (Glimmer DSL for SWT) to have people avert that error with a more user friendly experience than being required to upgrade their gem system version.

But, I just had an idea. I assume JRuby regularly upgrades its default gem command system version from time to time. So, I was wondering if JRuby was planning on bumping the included gem system version up soon to get rid of the incompatibility issue with Bundler 2.6.x by default. In that case, I wouldn't have to do anything in my project, or maybe I could do something temporarily until JRuby adopts a newer version of the gem system as a default.

@headius 's response is included for record:

yeah I actually ran into that yesterday and then it went away because I upgraded rubygems for a different reason. It's pretty annoying that they made a breaking change in bundler that's not very far ahead of the last rubygems version missing that method
Please open an issue for that and we will get things upgraded in 9.4.11, which should come out in the next two weeks