Either of the changes could be server-side. The icons, or something similar, could happen server-side. The New Posts restriction could happen server-side. Both could also be done client-side.

Apophis is a bottleneck if you go the server-side approach. His many other tasks, combined with the complexity created for any forum maintainer when introducing any modification to the site (assuming he even sees enough merit in the mod to install it), adds some (conditional) incentive for managing a client-side solution whenever it's possible. That would be one argument for managing the New Posts restrictions client-side, I suppose. You don't have to wait on Apophis, and his forum maintenance complexity increases none.

Cing pointed out that he'd like his own icons. When I recommended the server-side mod for the New Posts restrictions, it didn't occur to me that each user might like different forums displayed. That's another argument for managing that tweak client-side, as well, I suppose, unless a server-side mod could be introduced which gives this freedom to each user (unlikely with icons).

The control that client-side modifications give you is nice, but things look so much cleaner if they're done server-side, typically. Yeah, this icons thing doesn't impact performance much, but you're going to see some (arguably not that problemmatic) screen flicker if I design a solution which restricts New Posts to certain forums, as the browser must receive and render the unwanted thread rows before my userscript can then "immediately" remove them.