So, I did some experiments with various versions (5.32.1, 5.40.1, 5.52, 5.60.2, 5.70.1 and 5.81)
And what I can see is that the beefiness started with 5.4, all the way through to 5.70.1, but from my perspective it's gone in 5.81.
5.81 looks apart from the letter spacing close to the 5.32 version.
What PDF Viewer are you using?
It might just be a "perception" issue. Since the letters are made up from dozens of small, often parallel, polylines the scaling algorithm of the PDF viewer might try to show separate lines as long as possible before it is combining them into one pixel width lines.
BTW, the view priority in 5.81 is still different between Plan View, Sheet View and PDF for different drawing entities.