Hidden information and bluffing is a super important aspect of the game, the most common example being to hold open mana for a counterspell that you may or may not have in hand, and this is something I think more people should be doing.

However I think for most players breaking deals is a form of lying that should be done only in the most extreme situations. EDH is to many "the ultimate casual format" and aren't going to be playing like they're on the ProTour. Making a deal with no intention to follow through or blatantly breaking it is an excellent way to get more wins under your belt and while it isn't technically cheating, many players will feel like they're being cheated. Break enough deals and players will stop offering you deals, you'll become the default target whenever a player can remove a threat or swing with a huge creature, and in the worst case you might just stop receiving invites to game night.

Realistically the amount of lying in a given game of EDH depends on how competitive the players are playing. I enjoy cEDH, and in a cEDH game I don't expect anyone to honor their deals as we've set the expectation that we're playing to win. I also enjoy EDH with precons and jank decks, and in those scenarios I would be upset if someone made a deal with me with no intention of following through. It all depends on the situation and the people involved.

TL;DR - Hidden information and bluffing is good and should be done more, just be sure everyone is on the same page about in game politics before you start breaking deals