If your RimWorld Workshop mods are stuck in a "downloading..." state after the latest update, nothing else has worked (clearing cache, verifying files, reinstalling) AND you would like to not have to unsubscribe from mods causing issues, this worked for me.

Right-click RimWorld in your Steam Library → Properties

Go to the Workshop tab (near the bottom, 3rd from the bottom above Privacy and Game Recording)

Find the mod(s) stuck downloading and uncheck the box next to them

Then recheck the box to force a redownload

This worked for me when all other fixes failed. It lets you keep your mod list and disable incompatible mods without unsubscribing.