I believe the Avast signature only appears in emails sent or received via a email client, such as Thunderbird or Outlook (not webmail, though).
No, avast can insert it in webmail as that goes through the web shield not the mail shield. That was introduced a few program updates ago and there were many complaints about it being inserted into web mail.
Since it had never been in web mail when people started seeing it they wanted a way to disable it as disabling it in the mail shield didn't work. That is where the new setting was introduced in the avastUI > settings > General, so users didn't have to disable it from two different locations.