EPILOG: Sometime in mid-February 2016 Avast re-configured VPSUPD4. My guess is they deleted the low-threat virus definitions. As of 3/1/16, it has been trimmed-down to about 110Mb; it was this small back in early 2014. This makes it once again usable in Win98/ME. And, the smaller database will make the OS run faster than just before 10/3/15 when Avast48 balked. Doing an online update works as well, although if you haven’t updated for a while best to use VPSUPD4.

I was still using Avast48 on my Win2K build, and discovered by chance that the database size had dropped.

Given the rate that VPSUPD4 had been growing lately, it will be around 2019 or later before the database exceeds 200Mb and becomes unusable again for Win98/ME. Thank you Avast for the reprieve!