How do they do this on a vanilla server?

They aren't actually "vanilla". ;)

That is from: https://umod.org/plugins/save-announcer

Yes, I just found that out by trying /padmin and /vanish in chat.
How did they remove the Unknown Command chat message?
Is it as simple as removing this line from lang\en\RustCore.json?

  "UnknownCommand": "Unknown command: {0}",​

runescape

Yes, I just found that out by trying /padmin and /vanish in chat.
How did they remove the Unknown Command chat message?
Is it as simple as removing this line from lang\en\RustCore.json?

  "UnknownCommand": "Unknown command: {0}",​

They are abusing the Community by faking being vanilla. I would advise against it, as only admin tools are allowed under the Community section so you'd risk being blacklisted. The "Modded" setting is what toggles that though under oxide.config.json, but please don't abuse it the list.

I know how to make a modded appear under Community and I know FP has the right to blacklist IP addresses that abuse the «"Modded": false,». I'm just curious if removing lines from RustCore.json completly disables the related messages.

runescape

I know how to make a modded appear under Community and I know FP has the right to blacklist IP addresses that abuse the «"Modded": false,». I'm just curious if removing lines from RustCore.json completly disables the related messages.

No, the setting above that I mentioned does.

The setting «"Modded": false» is what hides the Unknown Command message?