Planning "as late as possible" depending on successor element

Without defining an end date, as late a possible woul plan to finish on 31/12/9999.
Ist'n it a bit too late ?

But to really answer your Question, what you try to get is a start-end dependency.
This does not exist in Projeqtor, only end-start exists.

Am I right, that such start-end-dependencies are not simple to implement?

Yes, it's quite hard, moreover in the way ProjeQtOr calculates planning : without overloading resources.

For instance, imagine 2 tasks A->B, A as late as possible depending on B start, single resource R assigned to A and B.
In the way ProjeQtOr plans, it should start to plan B, to get start date. But this will use R availibiity, so leaving no more room to place A before; this would involve to plan A before B... unless B has a constraints pushing it a bit later in the planning.
But if now another resource S is assigned to A, problem may be completely different as S may have availability to work on A... or not (depending on other tasks).
And these are simple examples.
Taking into account all use cases will really be a mess and will possibly (I'm afraid it will) not give expected result in most cases.