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?
Like already mentioned here, back up first. I use a tool called Foxclone for hard drive image backups. Many would recommend Clonezilla, but I can vouch for Foxclone as I know it works from personal experience and the interface is really intuitive.
If you installed your Linux distro to dual boot alongside an existing Windows machine, you might need to move the Linux partitions after deleting Windows in order to resize the Linux partitions. This is something that can be done later once you know everything is working okay.