This is no suggestion (that I have seen) that avast 5, when released will not support the Win9x systems.
However, at some stage in the lifetime of avast 5 it will be likely that the team will want to make changes to the update structure (as they have in the past). Also in the past they have needed to force updates of avast itself to accommodate those changes. Beyond 2009 they are not going to work to make such changes compatible with the Win9x systems.
So, when that day comes it might occur that you could not update the program on Win9x systems and the updates would no longer be available for the older program releases.
As Vlk has commented, they wanted to make sure that users have plenty of notice that Win9x support for avast will end.
As you probably know Microsoft is terminating the sale of Windows XP licenses in January 2009 (there is a little fudging where they can be ordered by then but do not have to be taken up until May 2009). After that Microsoft will only sell Vista until Windows 7 is released. It may be that you could hang on with the current configuration until Windows 7 comes out but with the risk of not being able to keep more and more products on your system up to date.