From December 2018, I will no longer be a beta-tester. As result, I have not been included in the talks about the development of Avast 2019, but Avast 2018 will be the latest version supported by Windows XP. Avast 2018 will receive security updates and bug-fixes, but XP users won't get the chance to upgrade to Avast 2019 and future versions. Which means that, even though Avast is an already really complete Antivirus, new features won't be included in the XP version. This is not something new, as all the older version of Avast still receive security updates, that's why Windows 98 users can still install Avast and get definitions updates on a former version of the Antivirus. The XP beta program is gonna end soon and the 18.x.x is gonna reach a stable version soon and will be updated to fix all the bug.
The same applies to Vista.