Built-in source code integration support has been removed starting from version 1.3.0., see https://mantisbt.org/blog/archives/mantisbt/440
Reintroducing support in Mantis core will hardly happen, as starting from 1.2.x, using the source control integration plugin was the recommended method.

The best way in long term would be to implement it as enhancement of the plugin.
The first step would be to create a ticket at https://github.com/mantisbt-plugins/source-integration/issues
I don't expect that someone will start implementing it (you never know), as CVS is dead since more than 10 years https://savannah.nongnu.org/projects/cvs so the second step would be to create a fork of the plugin and send a pull request with the changes.

While more thinking about it, I don't even know if extending the plugin is a good idea.
Implement something new (and maintain it!) to support something dead.
Wouldn't it be better to invest time in migrating CVS to Git or Mercurial?

Thank you for your reply and the pointers how to proceed. I think you are right: It would be better to migrate to a newer version-control system. On the other hand it can be quite costly (or even impossible) to migrate legacy source code with a legacy development environment (running on a legacy system for legacy customers) to a state of the art environment...

Never the less: The blog entry mentioned before (first hit on google: https://www.google.de/search?q=mantisbt+cvs) has a date of July 2007, so everyone can see, that this is quite old.

The FAQ (last changed 2014) is another story. Wouldn't it be a good idea to update the FAQ or (at least) mark the complete FAQ as outdated?

For that reason I put that ticket to category "documentation"...

Wouldn't it be a good idea to update the FAQ or (at least) mark the complete FAQ as outdated?

Right.
There is no clear decision at the moment, if the Wiki will be completely removed or updated and/or replaced by another type of documenation.
As a first step, I added a hint at the top of the page and removed the broken CVS links

migrate legacy source code with a legacy development environment (running on a legacy system for legacy customers) to a state of the art environment...

In this case I would keep Mantis 1.2.x, but update to latest 1.2.20 in case you are interested in having fixed at least some security related bugs.
If security is important for you, even 1.2.20 can't be recommended.

Thank you for the updated FAQ, much better now.

May I point you to another outdated page: https://mantisbt.org/wiki/doku.php/mantisbt:features?s[]=cvs

May I point you to another outdated page

Thanks for the pointer, but I think there is no need to start a list of outdated pages.
I expect it could become a quite long one :-)

Closing as won't fix. As explained, CVS is dead and we have no intention of restoring the legacy source integration.