And to answer the original question, it's been like this for quite long; I'd say at least 3-4 years - that's how long I've been paying actual attention to Steam, and I don't remember ever seeing a different icon for Linux.

And having been on steam since The Bad Old Days of Wine (not all that long ago, really), I can say that we used to have a penguin, and then it got replaced with the steam logo a few months after steamos and the steam machines went on the market.