[2022] old savegame compatible with 1.6 ?!

No. Outdated save files will load in 1.6, but due to map and history files changes in 1.6 you will eventually run into stability issues.

It's strongly recommended that you start a new game for 1.6 or roll back to 1.5.x to continue your existing campaigns.

Originally posted by Ratlegion: Wasn't it explicitly stated that the new update was compatible with old saves, ideal or not?

Major updates are essentially guaranteed to break save compatibility, so that's unlikely.

Rule of thumb is to assume that your saves will be incompatible with any other major version of the game (eg: 1.5.x and 1.6.x).