When attempting to delete a string table, I found that a UserWidget was still referencing it. The UserWidget only contained 2 text blocks and no text properties, both text block texts were already unlinked from localization. Odd. Then I saw it:
The textblock widget still shows “Placeholder” next to the widget icon on the widget hierarchy panel. Which makes no sense because its text property is unlinked and I typed a culture invariant text “MyCompany” in it.
Changing the culture invariant text made the text on the widget hierarchy panel update. After this the string table was no longer referenced. Apparently there is a garbage reference stored in UserWidgets somewhere other than textblock fields.
I don’t know what triggers it but I had to modify text properties on about 20 UserWidgets to fully dereference the string table before its deletion. Must be a common trigger then.