Because themes are updated independently of Core updates, any deleted files from bundled themes should not be included in the `$_old_files` list.
Any file included in this list is deleted on update, which could cause problems for sites with a given theme active if the removed files were required in earlier versions of that theme and that theme is not updated at the same time.
Props desrosj, costdev, SergeyBiryukov.
Fixes#56936.
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