The structure of the directory as it stands is based on work that had already been started by the community here on the forums. None of this should be new. We already had two categories well before the directory started to take shape: Plugins Confirmed Working on ClassicPress v1 and Plugins built specifically for ClassicPress.
These lists were then assembled into separate spreadsheets for ease of import into the directory.
When the WP repo spreadsheet was being collated, a manual check was made to confirm that the plugin was a) still compatible with WP 4.9 and b) still being maintained.
So, everything on this list has been both added by the community and confirmed as working by the community.
The new directory will, using the wp.org API, check the repo for updates on a regular basis (exact schedule tbc). This will include checking the requires field. If this returns anything greater than 4.9, the plugin will be hidden from the directory and flagged for manual review.
One other aspect to consider is, as @anon95694377 is well aware, only plugins hosted on GitHub will initially be listed. Support for other platforms - such as GitLab - will be included as soon as is reasonably practicable.
Regarding those WP plugins for which the dev has confirmed support for CP (i.e. “Supports CP”), there was some talk of perhaps using a badge type system (and yes, possibly a filter) though I’m not sure if this will make it into the first release.
If we were to only show “Supports CP” plugins, it would greatly reduce the number of plugins available in the directory. This would exclude some great plugins such as Advanced Custom Fields and I don’t think that’s helpful to anyone.
Hope this clarifies things.
To sum up, I agree with James: