At the moment we enable both, onions (primary) and clearnet (fallback) apt sources.
Proposed change: disable (comment out) onions by default
Reasons: Enabling it was an experiment which failed. Unreliable, bad usability (many support requests), reflects badly on Whonix.
Once Debian provides onion v3 with onionbalance which works reliable we may reconsider.
Once reliable we may even implement:
use onion sources list exclusively for apt-get updating by default