Should it be removed (temporarily) for this release then?

Probably not. However that also means the campaign will be empty this time around ...

It is difficult to know when the Slavs separated from the Balts. The Kiev culture is the only one that can be definitively attributed to the Proto-Slavs. Regarding earlier cultures, there is no certainty. There are many contradictory and contested hypotheses. There is no consensus concerning their relationship with the Proto-Slavs. The cultures most frequently discussed are the Milograd culture, the Dnieper-Dvina culture, the Zarubintsy culture, and the Chernoles culture.  In my opinion, the Kiev culture derives mostly from Dnieper-Dvina (Днепро-Двинская культура).

The only new map is the sandbox map of the Germans. But I don't know whether sandbox maps were shown by that filter in the past.

In the past I remember a map filter for "new maps", but I cannot see it in 0.28-rc1. Are there no new maps in this release or the filter is just missing?