I'm almost 100% certain that it was due to be released in the European Automotive car pack, but they had to recycle in the Pininfarina Battista instead at the very last minute.
The only thing I can think of is that Maserati asked for something that Forza didn't want to accept in order to license it.
Let's not forget that Maserati hasn't had a single new car added since the Levante in FH4:
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No new version of the Gran Turismo, no Quattroporte. Both would be a great addition to the game.
Hmmm... Maybe Forza can magic up a licensing agreement for new Maseratis just like they did for Stellantis?