Just a word of caution - what appears to be deleted in Canvas is normally just an asset (Course, file, etc.) being marked as deleted through a status flag. Some content can be accessed via API or report even with the delete flag in place, and content can usually be restored by removing that flag. Essentially that is how undelete works. I haven't examined the course purge button, but typically in the UK we have a six years academic record obligation, which includes the LMS.
If sensitive content needs to be permanently removed, it would probably take a conversation with Instructure to achieve that.
I do generally recommend to colleagues on any LMS that they make a habit of downloading their own SCORM export of their site for 'later career use' and in the case of Canvas a SCORM download from an institutional course can normally be reloaded to 'free for teachers'.
@lotter I'd recommend that you have a discussion with your school or department to see if a colleague is comfortable downloading your old courses as scorm packages and sending them to you (this doesn't include student work or grades unless you have 'hard copied' old student work into your content). It may be that the permissions haven't been set to allo that, but your School may be prepared to have a discussion with Canvas Admin/ IT to facilitate that.