Existing buildings on plots in a new development plan

is the topic still current?

Oh, so you just wanted to know what could possibly be placed in the area marked with crosses??? Did I understand you correctly?

Yes. Since it is explicitly mentioned here that nothing is to be built there, this is, in my opinion, a fundamental feature of the planning, so that exemptions according to §31 of the Building Code are excluded.
The only remaining option for someone wanting to build in the garden would be to persuade the municipality to repeal or change the development plan. Such a fundamental change to such a new development plan is extremely unlikely. And even if it came to that, you could signal strong opposition during the early public participation, so that the municipality is no longer willing to pursue the plan.

Absolute certainty, of course, only exists if you acquire all neighboring plots.