Since the proposal message was cross-posted to several mailing lists, the follow-up discussion is a big mess, but you can wade through it if you're sufficiently interested: Search OOo mailing lists for "drop Win98 support".
The upshot is that it costs a lot of time and effort to support Win98/ME, and even with that effort the support is not as good as for the other platforms. MS is not updating 98/ME, so OOo has to emulate new OS features and work around bugs that are fixed in newer Windows.
Once OOo 3 is released (September 2008), the choices for those using OOo on Win98/ME will be (by my limited understanding):
- Upgrade to a newer Windows (that pretty much means Vista and new hardware), or
- Upgrade to a different OS (Linux or Mac), or
- Continue to use OOo 2.4 and accept minimal support