I don't think this function ever cleared the contents of the text based logs (even in previous avast versions), it merely cleared the entries in the avastUI list. The nicely laid out information in the UI is obtained from the text based log files.
The information for the scans listed in the UI Scan History could be coming from multiple files, one for each different scan that you do, Full System Scan, Quick Scan, etc.
I don't know if you are trying to recover some disk space or not, but these text files aren't going to be that big. I have 9 .txt files in the <...>AVAST Software\Avast\report folder with a total of 450KB.