Originally Posted by andrewesque

Unfortunately I believe this is no longer the case. The last stop for the TXL bus is now just past Berlin Hauptbahnhof, and the bus no longer runs to Alexanderplatz.

From the BVG bus schedule (PDF, in German):
Ab. 4. August 2019:
Linienverkürzung. Die Linie wird vom S+U Alexanderplatz zum S+U Hauptbahnhof (bzw. zur Haltestelle Robert-Koch-Platz) zurückgezogen. Den entfallenen Linienabschnitt übernimmt die Linie 245.

From 4 August 2019:
Line shortening. The [TXL] line will be withdrawn from S+U Alexanderplatz to S+U Hauptbahnhof (or to the stop Robert-Koch-Platz). The canceled line section is taken over by line 245.

It's subtle, but the BVG network map has also been updated to show the truncation of the line.