Specifically, in the previous version 78, text in the UI (toolbar, etc.) and text in the folder/message panes all had subpixel anti-aliasing.
In version 91, the text in the UI continue to have have subpixel anti-aliasing, but the text in the folder/message panes just have grayscale anti-aliasing.
Here's a screenshot illustrating the difference; the image on top is from TB78, and the one below is TB91.
Here's a zoomed-in view; note that the anti-aliasing for all parts of the TB78 part of the image are "colorful", which is the tell-tale sign of subpixel anti-aliasing, whereas for the TB91 part, the anti-aliasing in the toolbar is "colorful", but the subject text is 100% grayscale.
