MarsSeed [1] filed a deletion request for heifthumbnailer [2]: Obsolete KDE5/Qt5-only thumbnailer; EOL/unmaintained since 2020. [a]
KDE is now based on Qt6, with legacy Qt5 support. It already has a built-in thumbnailer for HEIF/HEIC files via kimageformats (Qt6) and kimageformats5 (Qt5). [b] [a]: https://github.com/zzag/heifthumbnailer [b]: https://archlinux.org/packages/?q=kimageformats [1] https://aur.archlinux.org/account/MarsSeed/ [2] https://aur.archlinux.org/pkgbase/heifthumbnailer/
