A simple web server can hold millions of connections. With some smart caching you could serve a wide audience. If the DDOS is big enough you will max out your 1GB/s line - I'm not sure on ways around that. Even blocking ranges on your router won't fix it.

This is not too hard of a problem, but the issue is that it's expensive. You can likely get to ~100k packets/second of forum serving + DDoS attack filtering on one core, but you will eventually saturate your (likely 1 gbps or less) upstream connection. The only real mitigation to this is to have a much bigger upstream connection than you need.

Edit - clarifying that 100k pps per core would only be achievable if the vast majority of the traffic (>95%) could be filtered out.

They might look into a federated server + p2p content exchange architecture like notabug.io was building for a few years.

Something like BitTorrent wouldn't work well for dynamic content such as a web forum.

There is live streaming built on top of Bittorrent. Sure you have to tweak it but it is very possible.