Many employment contracts are structured that way. You basically are on a 90 day contract perpetually. That would be the period of notice they would have to give you terminate you without cause. If an employer were to change your salary, I would imagine that you would be offered a new contract to sign. If you decided not to sign then you would either continue with your current contract or the 90 days would expire and you would no longer be employed. The real question in the scenario of a non-renewal of contract is of any non-compete is enforceable.
Most employer cost cutting is usually more insidious than they saying “hey, we’re cutting your salary.” It’s usually in the form of staff cutting, workload increases, non-contractual “bonuses” being taken away, etc. They maintain the contract you sign, but things like lifestyle and other perks slowly degrade.