I'm looking into getting a 98SE setup going again and am trying to find the latest Avira update. According to this link support ended 31 Dec 07, but then 'maintenance' removed the 9x/NT update files only a few months later in early May.

I've been looking and the latest I've found (http://dl.antivir.de/down/vdf/ivdf_fusebundle_9x_en.zip via WayBack Machine) seems to be Mar 21 2007. I have also found another database which is supposedly Jun 19 2008 (also listed as 30 Jun 08), although I don't trust this because apart from the fact the files are dated after their cutoff date, the filesize is about 50% larger than the previously mentioned one.

So did anyone happen to grab a later copy before it went down?

You can try this: Download the latest definitions link. Open the zip file with 7Zip or another zip app and paste the four vdf files in your Antivir app folder.. Don't paste the avpack32.dll, because it doesn't  work anymore on 98/ME. The last avpack32.dll that works was released on 29.04.08.