It has been said in the forums before that the intention is to continue support for win9X. However there are likely to be new functions that won’t work with win9x. This is not a limitation of avast but of the OS.

Like the Network Shield doesn’t work with older OSes, this shouldn’t be a problem as you should have a good firewall installed in any case.

Network Shield provider protects your computer from Internet worm attacks. It works similarly to a firewall, even though it does not fully substitutes it. The Network Shield does not require any user interaction.

It is probably because of limitations of the OS for new functionality which won’t work with them. You are going to find more and more of this even MS don’t support win9x.