I really wanted something similar so I threw a quick and dirty tampermonkey script together to fetch the subscriptions from the settings page. I think it worked out pretty alright.

Edit: Someone else can probably do something a lot better, but here is the code: https://pastebin.com/P0mR9QL6

Paste it into a new script and call the addSubscriptionsSidePanel() function to run it.

Edit2: I’ve uploaded it to Greasy Fork so that you can install it directly: https://greasyfork.org/en/scripts/469139-kbin-subscriptions-panel