I know Venice was a city of Hermetic intrigue in 3rd edition, but have we ever mentioned that in 5th?
Venice has not yet been mentioned in ArM5, but I myself basically consider old cannon to still be valid until something newer contradicts it (well, except for that Flambeau bit). As for the Roman Tribunal, I think the only reference made so far is the tale of Varidian's Tomb, found in Ancient Magic.
As for Venice, I myself would be very dissapointed if 5th ed transformed it as to not be a city of intrigue. It is just a long established part of Ars Magica tradition. However, transforming it into a different sort of intrigue, that could be interesting. Retconning is fine as long as landmarks are observed and a nod is given to what came before. So, what sort of "different" intrigue could work? I myself like mixing magi into history, and thinking of the Order as part of history. Things end up they way they do in 1220, and the magi are part of the reason things worked out that way. Far from the idea of "conspiracy-history", I view magi as inescapably tied to history. They are still a part of human society after all. Perhaps as a subculture or even counter culture, but still a part of it. Getting to my point, just as magi influence mundane history, mundanes have influence the history of the Order. Taking it to Venice, I can envision all sorts of intriges involving magi manipulating society, and the power players of mundane society putting the squeeze on magi and influencing tribunal politics.
I don't believe this has been explicitly mentioned in AM5.
Covenants, p. 92, l. 6
C&G, p. 139, bottom
Venice has been mentioned, but not been discussed in a major way afaik.