I think PUP should be enabled by default. It would be good for majority of users i.e average users.
Some PUP are very difficult to remove/uninstall & create probs for average users.
But users differ in their opinion about PUP.
I think in Avast PUP should be enabled by default for average users sake.
For advanced users sake Avast can set the PUP settings in RealTime as ask & OnDemand as select/choose in result window. Good for both average & advanced users, wot say?
When setting up scans, there is a turn on/off button for scanning PUP. Exactly what is this.

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