The net effect was you could make your own news feeds / timelines and use code to control how they were filtered / combined / etc... It was crazy powerful (for 2005) and I still miss it _today_ since it had the dynamism of the news feed, some of the social aspect, and total control since there was no algorithm other than your and the people's who's list you subscribed to curation and any code you ran against it.
Not a lot left from those long ago days but I did find one slightly-cringy video: https://www.youtube.com/watch?v=45DSrU23sPI
:)
Unfortunately, this isn’t supported by the majority of RSS clients (tt-rss is the only one I know) which means in practice OPML feeds are merely import/export mechanisms.
https://techcrunch.com/2006/09/18/grazr-10-blasts-off-into-t...