Keep just one revision within a time interval periodically

Added the plugin and some tests.

Do you think I should add more integration tests with all the 4 modules working together?

Let me check, the "active" revision is always kept, so this plugin will keep 1 extra revision, but only if it is not older than a specific period? So if I specify a period of 1 month, we have the following:

• Latest revision: today
• Previous revision: 3 weeks ago

Both revisions are kept.

• Latest revision: today
• Previous revision: 5 weeks ago

The previous revision will be deleted and we only have the active revision.

Is that a correct representation?

To be honest, I think this is one of the reasons why we moved to plugins. This sounds like a very specific use-case, and adding this to the main module would add complexity for all users, which we should probably not want. I think there are a couple of options:

1. This could easily be added to a custom module, in that case this is up to you (or others) to implement and maintain.
2. We could potentially create a new project, a "node_revision_delete_extentions" module with a bunch of submodules to add plugins for advanced cases like this. That would keep the main module simple, but still allow non-developers to download/install the new project and take advantage of the Drupal community for more advanced cases.
3. We could even add advanced use-cases as submodules to "node_revision_delete", but I'm not sure if the maintainers are willing to support the submodules and all the work that comes with maintaining them.

For now, I'm leaning slightly towards option 1, and if someone is willing to maintain this, option 2 is also fine with me. We should probably validate the need for certain plugins through new projects, before we add more complexity to the main module. When a project has lots of usage, we could always decide later to incorporate it in the main module later.

Yes, your representation is correct.
And I like the option 2 because that might encourage others to contribute with their own plugins and that would help others at the same time.