HTTP / HTTPS:
After checking with the Launchpad channel on IRC, only private PPAs get HTTPS, under the current system. Public PPAs do not get HTTPS connectivity methods.
Manual Downloading Method:
Your only other option in this case to get PPA packages via HTTPS for public PPAs, here, would be to manually download the packages off of Launchpad (via Launchpad Librarian), by navigating to the PPA on the Launchpad interface, hit the "View package details" section, expand the given package version, and manually downloading all the .deb files from Launchpad, which will use HTTPS. This, however,
FTP / SFTP / FTPS:
None of the PPAs support FTP/SFTP/FTPS download mechanisms. You cannot use these methods to download packages from PPAs. (FTP and SFTP are only available for uploading via dput, and are not available for downloads).