I have a dual boot Win 10/Debian setup on my laptop. I have not touched the Windows boot in months. There is nothing on there that needs saving.
How can I ditch the Windows partition and make it available as free space in Debian without damaging my Linux setup?
Depends what method your doing.
Dual disks you can do a raid configuration(if the disk is formatted as btrfs or any file system that supports creating raid without formatting,it can be done through ext4 but needs formatting ).
If the windows partition is on one disk.
Make sure the windows partition is adjustent, use something like gparted to merge it.