I'm not aware of "new trend" or "complicated speech".
The so is here simply used as a reference, compare "so" on DWDS, III.3.:
Grammatik: leitet einen Rel.satz ein, bezieht sich nur auf einen Nom. oder Akk.
Der emeritierte Herr Professor, so seitens der alma mater mit den Vorarbeiten zur 500-Jahr-Feier beauftragt worden
The example there ...
Mostly a misunderstanding of a quite ugly sentence:
das is the article of Ministerium and the verb (something like: kommentierte = commented) is omitted. Due to the ambiguity of so this provides no clear indication, that indirect speech is following, but leaves that for the reader to detect. (Integrated my comment with that of O. R. Mapper.)
The quote in ...
Here's another (double) example. In a humorous column in 'Die Zeit' on 11th November, 2004, Harald Martenstein wrote:
'Berlin geht so was von den Bach runter. Berlin ist so was von fertig'.
It seems to me that a good idiomatic translation of the phrase here would be 'as good as'. (This also works for the example 'so was von tot'.)
Short answer: No, you cannot say Altestadt instead of Altstadt.
Long answer: You encountered a composition of an adjective and a noun.
First, even though such compositions may have been derived from phrases in that both the adjective and the noun stood alone, compositions have developed their own specific meanings. For example, die Altstadt refers to the ...