If you didn't know bitcoin had a warning system like this, that's because it was retired in 2016 due to security concerns and frequent confusion about its use.
"The alert system was a frequent source of misunderstanding about the security model and 'effective governance,'" well-known Bitcoin Core contributor Greg Maxwell wrote in a public email from September 2016.
No kidding. It's a relief we can put this behind us. Several years on and I'm still mildly traumatised from the levels of ignorance and fear-mongering on display in
threads like this. Some people just genuinely didn't "get it".
Incidentally, was that thread the main catalyst for this decision? Would be nice to know that something positive came out of that mess.
Interesting to see that a potential DDoS vulnerability was found, though.
The network is not at risk and this warning may be safely ignored if you do not have an ancient node (running v0.12.x or older) using the deprecated bitcoin alert system or its public keys.
I'm thinking the hardline fundamentalists running deliberately outdated "TRB" nodes might need to fix theirs?