An active subscription is required to access advanced features such as editors. When your subscription expires, these editors will enter into read-only mode, so you can continue to see your data with the visual markup provided by the editors, but not use the editors to modify data.
Because editors are typically visual layers on top of your existing data, you can simply disable these editors to go back to editing your data using plain means. For example, if you were using Markdown Pro to edit a note, this editor will enter into a read-only state when your subscription expires. You can then simply choose the Plain Editor to edit your note as usual.