No-one is ever told explicitly to take it easy but the focus is completely different to a premiership game so it's essentially implied.
For example a key focus of the preseason game will be how many minutes you are playing. In a real game you will run rotations around the game, so if the ball is stuck and you can't get off you would keep going, in a preseason game you would just head to the bench regardless.
Sides will also hold back on structural things, basically run a cut down version of aspects like kick-ins, stoppages etc....They have already run these things at training and in club scratches so the work is done there.
The build-up is nothing like a real game either, there's essentially no opposition focus compared to usual, it's all about what the team needs to achieve in terms of preparation, normally it would be at least 70/30 the other way.
The intensity before and during is way down from everyone, often the assistants are coaching, there's more intervention from the conditioning team and there's an unquestionably different attitude from the players in terms of preservation vs intensity. The exception is young guys trying to break the side which is why you often see someone stand out like that (Hello Sampson Ryan).
In summary, it's nothing like a premiership game in nearly every way.