Is it mandatory to adopt expansion joints?

The parquet would be installed as a floating floor, right? So that it can additionally compensate for any movements of the screed underneath to the fabric laid below?

If the floor is firmly connected to the screed: leave an expansion joint. If the screed can expand under the floor: fabric solution OK

we have an old screed and next to it a new screed. The two were also "somehow clipped" together. On top of that lies oak parquet (20mm thick). So far (it's only been 2 years) everything is fine.

I cut open at my place, inserted screed clip and resin bonded. Floor covering vinyl 2.5mm glued

I cut open at my place, inserted screed clips and resin-bonded. Flooring vinyl 2.5mm glued

Common practice and proven thousands of times.