I have searched long and hard for a dictionary translation of "soft deadline" to no avail. Google Ngram shows that it has peaked in popularity in the last three decades, so it's relatively new. I think it comes from computer science.
In any case, does my translation follow? Any other ideas would be appreciated.
As requested in the comments, here is an example in which I would use this "soft deadline" expression:
"The hotel has a hard deadline for releasing rooms for the conference, so we should move our soft deadline for attendees back a week from that."