Most common theory is that they received multiple copyright strikes at once (considering the one Tails vid) and ontop of the fact they used an adblocker which youtube HATES. However there are rumors they're coming back this year so fingers crossed
Disclaimer: this is a fan channel and not an alt of the actual Furnace account. These were reuploaded on bilibili (by idk by who I don't speak Chinese) so when I found them I screen recorded and reuploaded them here for archival purposes, please go check out their actual channel in the description