https://w3c.github.io/epub-specs/epub33/core/#sec-container-filenames

File Names MUST NOT use the following [Unicode] characters, as commonly used operating systems may not support these characters consistently:
[...]
Tags and Variation Selectors Supplement (U+E0000 … U+E0FFF)

The i18n WG noticed that restrictions on Tags and Variation Selectors Supplement may prevent the use of emoji tag sequence (ETS). If the file system allows these code points to be included in file names, this may be a barrier to users or ends up with requirements being ignored.