The DocViewer in SpeedGrader will render HEIC files with no issue when they have been submitted by students as part of an Assignment submission.
However, when it comes to Quiz question response submissions, the options for HEIC files become more limited. Canvas does not consider HEIC to be an embeddable image file when students attempt to embed these file types into their Quiz question responses through the Rich Content Editor.
As HEIC is an Apple-specific file type, on PC machines, Canvas will not accept HEIC as a file type for student uploads within Quiz question responses.
On Mac machines, Canvas will allow students to upload a HEIC files, but only if a student chooses "All Files" instead of "Image Files." Even then, Canvas won't embed the image; instead, it creates an HEIC link in place of an embedded image, which can cause problems if an Instructor, TA, or Grader has to open the file on a PC.
In order to ensure that graders see embedded images, students using iOS devices to capture photos and images will need to convert these files to a format other than HEIC. Users can convert HEIC files on their iOS device by using the Files App. See these instructions: How to convert HEIC photos to JPG.
Otherwise, there's a chance that this will continue to be an issue for users attempting to grade HEIC files on a PC.