I explained why I was closing the enhancement request in my earlier comment.
Pipeline jobs are the preferred way to handle multiple repositories from a Jenkins job. They can be configured to support clone from a repository with self-signed certificates the same as freestyle jobs. Pipeline jobs can be created from the Blue Ocean pipeline editor. Support for multiple repositories is available from both declarative pipeline and scripted pipeline.
Regarding your comment:
The 'request' is also a bug report. The current way it works is broken.
I'm fully in favor of calling anything that "bugs" someone a bug report. In this case, I don't see any difference between whether this is a "bug" or an "enhancement". Whether this is a bug or an enhancement, Jenkins Pipeline is still the preferred way to handle multiple git repositories in a single Jenkins job. I don't intend to work on this. The other maintainers of the Jenkins git plugin (Fran Fernandez, Robert Sandell, Rishabh Boudhilya) are unlikely to accept a pull request proposing to do this and are even less likely than I am to implement this themselves.
Adding support for multiple git repositories to Freestyle, Maven, and Multi-configuration jobs would add significant complication to the already complicated code in the git plugin. That addition would only help the small set of users that want to use Freestyle, Maven, and Multi-configuration jobs with multiple repositories. Since Pipeline jobs are already over 50% of all Jenkins jobs and their portion of total Jenkins jobs continues to increase, I'm unwilling to consider implementing this.
Regarding your last question:
Is it only you doing anything with Jenkins source anymore?
I am definitely not the "only one doing anything with Jenkins source". The git plugin Google Summer of Code project we just completed included 4 mentors (Fran, me, Omkar, and Justin) supporting a student (Rishabh) that worked full-time for 3 months on the git plugin enhancements that were delivered in git plugin 4.4.x. There were 5 other Jenkins projects in Google Summer of Code, each with a team of mentors to support the student.
The Jenkins UI SIG has released theming support, including the dark theme plugin and improvements to the Jenkins plugin manager.
In addition to those efforts, the security team has released multiple security fixes for Jenkins core and for plugins.
The infrastructure team has made dramatic improvements to the Jenkins update center, has increased the number of Jenkins mirrors, and has switched to a new and much better supported mirror solution.