As recently discussed (also see #911 (comment)), the concept of modules will be entirely removed as part of the 3.0.0 release.

At this point, all features are already moved to standalone plugins exclusively, except for the Site Health modules where this was an explicit design decision, to keep features that don't directly enhance performance part of Performance Lab. This philosophy remains, with a slight adjustment of no longer using modules:

Any features that directly enhance performance should be standalone plugins.
Any features that don't directly enhance performance (such as measurement features or recommendations like Site Health) should be part of Performance Lab core.

Site Health checks should continue to remain part of Performance Lab. However, going forward, this should no longer be in the form of modules, but simply as part of Performance Lab core. This means:

• They'll be permanently active, as long as Performance Lab is active.
• There won't be a UI to control them. Their only UI will be the respective check itself within Site Health.
• They should still be implemented in distinct folders, in a standalone manner.
• They can still be ported over to WordPress core, once deemed eligible. At that point, they would be removed from Performance Lab.