I upgraded from Kubuntu 20.10 to 21.04. According to the answer at Any app on Ubuntu to open and/or convert HEIF pictures (.HEIC, High Efficiency Image File Format)? support for HEIC/HEIF is a part of KDE Frameworks 5.80, which is included in Kubuntu 21.04. Similar information is seen from the KDE Frameworks 5.80.0 release annoucement.
Nevertheless, after upgrading to 21.04, I can neither get thumbnails of HEIC images in Dolphin, nor can I open them in either Gwenview or Kdenlive. The only support continues to be similar to that before the upgrade, namely in GIMP and DigiKam. Strangely enough, a folder of HEIF/HEIC images does have some sort of thumbnail in Dolphin. But not the individual images. Right clicking on one only offers GIMP as an option to open it with. When I nevertheless try opening in Gewnview, the error message reads, "Loading 'so-and-so.heic' failed. Gwenview cannot display documents of type image/heif." When trying to add one to a Kdenlive project, the message reads, "Cannot open file so-and-so.heic."
Suggestions, please? Why can I not open HEIC/HEIF images?
EDIT: I don't know if it makes a difference, but the arch page for kimageformats 5.82.0-1 lists libheif as an optional dependency. The closest Ubuntu package is libheif1, which is installed for me, yet the Ubuntu package of kimageformat-plugins seems to have no equivalent dependency.