The implementation to make the sidebar a real sidebar on a mobile device will present a design challenge since the traditional approach would be a burger menu - but the burger menu is already present in the top right and introducing another burger menu could be confusing.

Facebook's an interesting example that has a top navigation, burger menu and bottom navigation on their mobile design.

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Currently, the primary navigation in the Feeds screen transitions from the left into a drop down. Whilst this is functional, it's a novel way of handling navigation (although the original designer of the site possibly didn't interpret the links in the left panel as the primary nav.)

With this in mind, for mobile, I could replace the existing hide-for-mq-large articles__header-select < Topic /> implementation with another new component which pins a (mobile style) tab bar to the bottom of the screen - therefore maintaining the key links in the primary navigation - It's a solution that I was also considering for a "top of page" navigation implementation and is something that a relatively new user (a professional UI designer from China) suggested to me soon after they registered:

For example, on the mobile side: It might be better if you can design four independent icons for homepage, community, message, and personal center and fix them at the bottom. So that users can quickly enter the interface they want to access during use.

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Currently, in the Profile view, the primary navigation runs across the top of the screen so for consistency, this would also require updating to reflect the changes being made in the Feeds area.

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Other ideas that could be worth implementing on desktop are being able to show/hide the sidebar which when collapsed, would only display the icons (which a lot of interfaces now allow - perhaps this is what you meant by "real"?) as well as pinning it to the side of the screen so that when you scroll, you don't need to return to the top of the page to change section - which was also suggested by a UI designer:

it would be better if the layout of the left and right columns could always be fixed on the interface

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I also think that moving the "My subscriptions" to within the "My Communities" section makes sense which would declutter the sidebar and make the "sticky" implementation cleaner.

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I'm keen to receive input and suggestions so please let me know what you think of the above and any ideas that you have.