I copied a friend’s layout and I do shift on my left thumb (below B). Before that, I almost exclusively used left shift.
If you use a 40% keyboard, I strongly recommend this. Heck, if you have a split spacebar at all, I recommend this. I do still have an extra shift key to the left of Z that I use for certain keyboard shortcuts, but I find thumb-shifting incredibly comfortable for typing prose and code. I actually only have 1 layer for numbers/symbols because I can hold shift with my left thumb to quickly modify them.
