I just removed [almost] 100 year old windows and it was a enormous effort to remove the paint from the screw slots and back them out. For my future self (or whomever might remove and restore when I’m probably long gone) is there a technique that is recommended to fill the slot before painting so that it is easy to clear later? I was thinking linseed oil putty but maybe something synthetic that is guaranteed not to dry out?
Painters Putty is commonly used to cover nail and screw holes.
Remember in most cases of construction the end product is intended to be in place for many years. The need to find and remove fasteners is of minor importance at that time. ( as you have experienced) Most craftsmen find no need to make easy removal a priority
This does not make the screw invisible, there will still be a small bump where the cover is. But it looks better than a screw. They're available in various colors and screw head types.
If you countersink the screws you can use flat covers:
Or you can just make them visible:
Now if you don't want to use screw covers, you can fill the head of the screw with something easy to remove, like acrylic caulk. It will remain elastic, so you can pull it out with the blade of a box cutter easily enough, and it's paintable. However the paint will form a hard layer on top of it, which you will still have to scratch off.
Linnseed oil putty will harden (although slowly) so it's hard to remove, and it is oil based, so acrylic paints will not have good adherence to it when soft. When applying this putty it tends to spread all over the place, and fingers get full of oil too, so you may end up with paint peeling off in random places wherever you spread the oil.