The asterisk, colon and underscore forms of gendering (Zuhörer*innen, Zuhörer:innen, Zuhörer_innen) have become popular in academia and in left-leaning social/political environments in the last 2-3 years. They're also used, albeit a bit less commonly, in public broadcasting ("öffentlich-rechtlicher Rundfunk") and public service and governmental work. Some authorities and government bodies have recently adopted internal rules about how to use gendering.
There's also a much older form of gendering with a capital I in the middle (ZuhörerInnen) that has been around for decades, but proponents of the newer forms see it as not including non-binary gender identities.
Particularly the newer forms have become extremely politically loaded, with conservatives often opposing them strongly, and progressives often framing them as inevitable progress towards more gender justice. Arguments of both sides can be found in the extensive and hotly contested German Wikipedia article "Geschlechtergerechte Sprache".