Update:
Nah it sounds like it’s fine actually. Here’s a statement we recieved from Valve:
Yesterday we were made aware of reports of leaks of older text messages that had previously been sent to Steam customers. We have examined the leak sample and have determined this was NOT a breach of Steam systems.
We’re still digging into the source of the leak, which is compounded by the fact that any SMS messages are unencrypted in transit, and routed through multiple providers on the way to your phone.