I deleted a custom field from the library. As expected, it no longer appears in Library & Folders > Manage Library Fields... > Show fields..., and it no longer appears in the Tag Tree (Tag Dump, All Fields, etc.). However the deleted field remains listed in the Content Panel's Bottom List column headers selection list (RMB on Bottom List column header), as well as in any previously defined Bottom List column where the field value was formerly displayed.
While the deleted field may still show up in the Vorbis Comment Block (at least until the block is re-written), its value apparently is not read/retained by MC. It does not show any value in a Content Panel's Bottom List column. So let's de-clutter.
Request: To avoid confusion, please completely remove deleted custom library fields from the Content Panel's Bottom List column header selection list (RMB on column header) and their Bottom List columns if applicable. This should happen automatically whenever a field is deleted, but may require a "cleanup tool" to remove existing deleted field entries.
Wishful Thinking Note: I find that undefined fields in expressions are generally "benign" - they just do not get evaluated. But if MC could identify all expressions with undefined fields (or at least a subset) whenever a field is deleted that would make the expressions easier to find and repair.