In 2021, a common "piece" of sharing was the iCloud Link , which allowed users to send high-resolution photos or videos via a single URL instead of individual files.
While not a standard global resolution for every app, this specific dimension (760x1226) is closely related to how generated link previews and shared media during that period:
Select to generate a web URL for sharing. Paste that link into a message or email. Share content in Messages on iPhone - Apple Support