I started Renoise at 2.1. I look at an old but amazing demosong from 2.0 by nt called “Please let me be”. But in 2.1 and after, it’s gone! WHAT’S WRONG WITH THIS AMAZING SONG!? WHY WAS IT REMOVED!?
Is it really necessary for every demo song ever made to be included with all new versions of Renoise? Probably not, and I think it’s unreasonable to expect that. Quite a few other demo songs have been removed over time, so Nt’s is not the only one. I don’t see a problem with removing some older stuff to make room for newer content, especially if it helps to keep the size of the download within some reasonable limit.
Technologies in Renoise change too. No point in demoing ancient features in Renoise which are not even compatible with current software.
All renoise releases have had their own Demo songs. As 2.6 is compatible with 2.5 (except AutoSeek, which does not modify sound playback), there is no real reason to upgrade the songs. But on the other hand, just to freshen things up… Why not? 
Still I think that song expressed sides of renoise that other demos don’t. I miss that song too.