First things first: the Legacy-Client has been removed for good now and it won't come back.
While we announced it a while ago and did so again as part of the last release notes, we are aware that some of our veteran players are not happy with this change and had hoped it wouldn't become a reality. The main reason for the removal is that Legacy it is outdated and only used by a fraction of the player base, while still requiring equal amouts of maintenance work by our developers as the Standard-Client. This is taking up valuable time that we want to spend on the active development of Supremacy 1914 so we can ensure the game is as fun and engaging as possible.
In case you're one of the players who used legacy until now, please provide us with any form of constructive feedback on why you prefered Legacy so we can do our best to adjust the Standard-Client accordingly, whereever possible.