Why is the Webflow CMS "locked" on some plans?

The "CMS" isn't locked on paid plans; rather, it's a core feature you add with the "CMS" or "Business" site plan. If your CMS structure (the fields in a collection) is locked, it's a technical safety feature. It means that collection is currently being used on a live page or in a Symbol. To unlock it, you must first remove all instances of it from your site so your changes don't break your live website.