In the old paint menu you just selected the pencil and changed the line thickness. Now the line thickness menu is dead unless you choose a shape, but there is no freeform shape option. So how do I change line thickness for a freeform line?
Click the Brush icon then adjust the line's thickness via the vertical slider on the left-hand side. You can do the same for the Pencil as long as you don't choose a shape.
Huh? I can still change the line thickness for the pencil tool just fine.
What irks me is that the choice of line sizes for other tools is now (Windows 10) only in odd numbers of pixels. It used to be possible to select 2 or 4 (6?) pixels for straight lines, rectangles, etc. I use Paint a lot and often find that I need just those missing sizes.