No. You may travel by plane, train, automobile, cruise ship, or any other method. You simply must comply with all Host Nation laws regarding that method of travel.
Previously, travel by certain means was forbidden or required approval at the O-5 level. This rule was designed to prevent travelers from being stranded. For example, if you caught COVID-19 during your trip, you may not have been able to board your return flight.
Now, airlines, cruise ships, and Host Nation authorities have sufficiently developed controls that mitigate the risk of infection (for example, increased testing requirements, or a requirement to prove vaccination, before boarding a cruise ship).
It is still possible you will become infected during your travels, and it is important that you plan carefully to ensure you can safely return. In many cases, ground transportation will still be the most practical means of travel, because you may not require a negative test to drive across a border.
It is your responsibility as the traveler to know the rules of both your destination and Germany, as well as individual commercial carriers.