It's probably left over old instances and updates that werent deleted properly. These apps tend to download the update as an entire bundle, rather than any sort of binary diff and patch.

Therefore, if you have an old install and have gone thru several (many) cycles of updates, the installation directory could be riddled with old versions and update files that make up the bulk of that 10 gig.

That is horrible, is it really that common that electron apps eventually consume all your storage with each update?

if the app cleaned up the update properly, then no. For example, signal desktop is fine.

I dont have discord, so what i said is just conjecture, but bad update files laying around isn't uncommon imho.