Yes driving on the shoulder is illegal and should not be done.
If you just need to pull over to the shoulder and drive slower to allow a vehicle behind you to get around you and ahead of you then that is usually okay.
Just be sure to immediately or as soon as possible pull back into the driving lane of the road or highway because the shoulder is not meant to be driven on.
The shoulder of the road is meant for people to pull off to check things and for people who are having mechanical problems with their vehicle.
The driver then pulls off to the side of the road which is the shoulder to get out of the lane of the road until they can get help.
So if you drive on the shoulder you might crash into another car that could be disabled on the side of the road.
I drive on the shoulder sometimes but only to allow a vehicle that is behind me to get ahead of me when they are driving faster and I'm driving slower.
Then I immediately pull back into the lane of the highway.
I've done that even when cops were around and never was stopped for it or ticketed.
I don't continue to drive on the shoulder though and only do so to allow a vehicle to pass me and then I'm right back in the driving lane.