There is currently 1 blocker for installing I2P by default in Whonix-Workstation.
Currently I2P (usual systemd unit file) is automatically started for users who upgrade from Whonix developers repository. And I don’t think we want to make all of Whonix users briefly connect to I2P during upgrade until next reboot for security reasons. I2P start was supposed to be opt-in, not default.
Autostart of privoxy and i2p systemd unit files gets disabled in anon-apps-config /lib/systemd/system-preset/50-anon-apps-config.preset, yes, but that package gets installed only after the i2p package is installed during the upgrade process. [1]
This could be handled in next release upgrade Whonix 15 buster → Whonix 16 bullsyee.
[1] That config file /lib/systemd/system-preset/50-anon-apps-config.preset is functional. It works for privoxy non-autostart but not for i2p non-autostart. It depends on the order in which packages are installed. Both, i2p and privoxy will not be autostarted after reboot.