Historically it was possible to insert invalid view definitions into system.views. If this happens, the view catalog goes into read only mode and it isn't possible to create/modify/drop views until the problem is fixed.

This was fixed in SERVER-43633 (MongoDB 4.7) and it doesn't seem any customer ran into this issue, but we still have all the associated code and tests associated for invalid views (forced via applyOps). Those are a source of increased development cost:

• Around 8 BFs by those tests even after SERVER-43633 was fixed - list.
• Long investigations like SERVER-72673, SERVER-74313, etc.
• It's not possible to upgrade/downgrade to viewless timeseries with invalid views present.

We should fail the feasibility check of FCV upgrade with a CannotUpgrade error if invalid views are present, after this is done we can remove much of the handling for invalid views in the next LTS release.