On my system it is here "C:\ProgramData\AVAST Software\Avast\report\Viruses & Malware.txt" and was last written to on 30/9/2020. I'm not aware of a change of location, which would seem strange given all other scan reirts are there.
I last ran a Smart Scan in relation to a topic in the forum, whilst compiling this reply I ran the scan and it did not update that file. So it looks like 20.8.2429 isn't updating that report file. I have checked through the minefield that is the avast settings including the hidden geek:area and I can find no such setting to generate a report file as there is for other avast scans.
By Omission or Bug I'm not aware, I had previously had the Viruses & Malware.txt, but it would appear for those installing 20.8 as a 1st installation the report won't be generated, nor updated for those who do have the Viruses & Malware.txt file present.