Yes, Microsoft supports product updates until the "End of Life" of a product. In case of Win98/98SE this date is Jan 16, 2005. They will stop supplying patches when the product enters it "Non Supported Phase" which will be 16 Jan, 2004.
I'm not sure if you can update Win98 through windowsupdate.com (most probably yes, but I'm running Win2k, so cannot check), but the Win98 site is still active (
click here) and it has a download section.