Is the Jaenisch/Schliemann Gambit being played or it has become out of fashion?
PS: I am a club player (FIDE ~1800) and occasionally use it to success.
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It depends what level you're talking about. At the top level with super GMs, it's more or less out of fashion in serious games. They either choose the Berlin or some other main line (like the Closed Spanish).
In the -2000 section though, it's still a viable opening. If you know it well enough then it's a good weapon to catch your opponents off guard. I would recommend learning another line to play against the Spanish though, so that you can alternate between it and the Jaenisch. If someone were to spend a serious amount of time preparing for you, they could cause some problems. If you have multiple lines though, it makes preparation much more difficult.
I looked up the Jaenisch's stats in the Mega Database, and it actually stores fairly well. White only scores slightly over 50%, and this is a normal stat for most main lines. Also, some top level guys have played it before (Aronian, Radjabov, Carlsen, Ivanchuk). While they don't play it regularly, it's still a testimony to the viability of the Jaenisch.
I'm not sure how many times it needs to be played by players of what caliber in what period of time to be considered "in fashion", but chess.com's database shows one game in 2017 where this was played and both players were rated over 2500. For 2016 it shows ten such games, including several 2700-level games.