Valve Patched Massive Vulnerability in Steam (that was there for 10+ years)

It's very likely that it has never been abused in wild.

They would still need to capture your connection to Steam first (eg. hacking your ISP, modem, etc.) to able to abuse this.
If they already can listen on your connection, being able to crash Steam isn't your biggest problem.

Simply UDP spoofing might be possible depending on ISP but:

"In order for an attacker’s UDP packets to be accepted by the client, they must observe an outbound (client->server) datagram being sent in order to learn the client/server IDs of the connection along with the sequence number."

If your connection is secure, they can do nothing. If your connection isn't secure, you're already fucked up in other ways.