I know this has been discussed before, such as in this thread but all previous suggestions seem quite messy.
Coming back to old songs I have thought of two ways I feel it could fit quite nicely into Renoise.
Firstly, as most of us are now using monitors with quite a high resolution, it could utilise the space left blank in the Song Properties tab. To the right of the Template Song box you would have an area which you can write in and it is specific to the track you are in, including Sends and Master (debatable if Master should show Song Comments.)
Second method would be to have it as a Meta device. Load it in the DSP Chain, type your notes in it, Minimise it so that it doesn’t take up much space unless you want to refresh your memory as to the notes you’ve left yourself. This would even allow multiple Notes per Track, so if you wanted to put some detailed ones for specific effects in the same chain it would be possible without having one huge note.
Either way I think could fit quite nicely into Renoise and hopefully make things easier for a fair few people.