In Ch22 Elizabeth criticises Lady Russell's style with the following comment:

”I used to think she had some taste in dress, but I was ashamed of her at the concert. Something so formal and arrangé in her air! and she sits so upright!”

This remark reflects Elizabeth's obsession with superficial appearances, but I like it also for the light it shines on Lady Russell. Assuming that LR was about the same age as her friend the late Lady Elliott, she was probably born around 1765. She would have been a young woman in the late 1770s and 1780s, and must be about 50 years old at the time of the novel.

The fashions of her youth were very, very different from those prevailing in 1810-1814.

The following link will take you to a Wikipedia page on the fashions of that period. In particular, check out the section on “Underwear,” and the fourth image down which shows the sort of stays which were worn. These were heavily boned with a long centre-front, and gave the torso a rigid conical shape. It was not possible to slouch in such a garment even if you wanted to. (Note: I have not checked every syllable of this article for accuracy, but the images are correct enough to show what I mean.)

http://en.wikipedia.org/wiki/1750-1795_in_fashion#Women.27s_fashion

From around 1795 to 1822 or so the high waisted “Regency” style prevailed. Stays were generally (though not invariably) much shorter, doing more or less the same job as a modern “uplift”bra. For this brief happy period women's undergarments actually allowed their bodies to bend and move in a natural manner, and fashion illustrations of the period reflect this. A good example is seen in the first image on the page linked at the foot of this post, which is from Cathy Decker's Regency Fashion Pages. It is from 1810, but similar slouchy poses occur in fashion illustrations throughout the period, and presumably reflect fashionable deportment of the time, or may even have influenced women's posture.

I think Elizabeth's comment shows that Lady Russell, though undoubtedly clad in fashionable garments, cannot give up the posture which was “correct” in her youth and adopt the fashionable slouch of the younger generation. It seems all of a piece with the fact that LR had “strict notions of decorum”.