The percentage share figures I post are from thinkbox, which has no archive. BARB does have an archive (so you can look back at any week), but it doesn't give % share.
I know occasionally I've posted them, particularly with regard to Strictly and IACGMOOH, as they seem to be about the only two programmes left now which have huge live audiences. I'm fairly sure that in terms of % share, for Strictly at least it's been pretty stable over the time I've posting the chart. I'd guess (and looking at the figures I posted above may seem to support it) that its decline is in line with the overall decline in viewing, whereas the soaps for example are all losing viewers at a rate quicker than that.