I just wondered after a talk with a someone from Austria from the usage of „net“ (he didn’t know this) like here:
Ich kenne das nicht.
Ich kenne das net.
Is this a phenomenon of whole Germany?
The wonderful resource Atlas der deutschen Alltagssprache asked about how an unemphasised nicht is pronounced in everyday speech and aggregated the result to give the following map:
As you can see, Germany is essentially North/South split on this pronunciation (like many others). Net and its cousins nit, ned, niad, nidda, nöd and it can be heard all across the South and southern centre with the northern boundary being approximately from the Hunsrück to the Saxony/Czechia border. North of this line, the predominant form is nich, sometimes nisch or nech (the latter is not separately distinguished in the map). Saxony itself presents somewhat of a special case as the dominant form is ni which lacks both the /ç/ sound and the /t/ sound.
The stray ni dots in the far North are slightly confusing to me and the accompanying text does not go into detail. This could be residual influence of the northern Platt dialects or somehow related to the proximity to Denmark (although in that case I would expect them to be even further north).
The shift from nicht to ned, net, nöd and nüt is a phenomenon that occurs in many mittel- and oberdeutsch dialects (so, roughly, in all regions south of the Main river), that includes Austria and Switzerland. See Grimm's Wörterbuch
Obviously, dialects tend to sneak into spoken Hochdeutsch every now and then, so you might hear "net" everywhere there.