This feature was exclusively part of a joke version and is not currently planned for addition to regular Minecraft. It is documented here on the wiki due to being officially-released content by Mojang Studios, even if not present in a final version.
The cursor is a joke block introduced in Java Edition 20w14∞ with an animated texture that changes its color every second, toggling between lime and black concrete.
Cursors are available in the creative inventory, and naturally generate in the "terminal" dimension and sometimes in other dimensions in Java Edition 20w14∞.
Breaking[]
Cursors require a pickaxe to mine.
Natural generation[]
The block naturally generates in "terminal" dimension as the blinking "_" next to a "C:\" made of lime concrete. It appears to be cycling between black and lime Concrete in appearance, which is what the rest of the "terminal" dimension is made of. It can also generate in the "skygrid" dimension, along with all other blocks.