Can someone explain what is this 90 day restriction about

You can read the whole thing for yourself here.

View and remove your associated devices in iTunes - Apple Support

When you try to associate another Apple ID with a device that was already associated, you might see a message that says, "This device is associated with another Apple ID. You cannot associate this device with another Apple ID for [number] days." Depending on when you associated your device with your Apple ID, you might need to wait up to 90 days to associate the device with another Apple ID.

As LACAllen said, there is no way to get it off your phone. You have to wait until the 90 days have elapsed before you can sign into another (your's I hope) Apple ID.

You would have to provide some context... just referring to a "90 day restriction" doesn't give us enough to go on. 90 day restriction for what?

If you're referring to the 90 day restriction for switching the Apple ID you use for the iTunes Store, one proponent is that it's a piracy prevention measure. Otherwise people could just "share" a.k.a. pirate music with other people who do not have a license to that music, and then just switch back and forth between Apple IDs.

It means you have done one or more of these things...

There is no "getting it off your phone".

You have full use of your device expect for iTunes.