Are shoulder charges and other dangerous tackles allowed?

No. A shoulder charge is misconduct under Section 15 of the International Laws (law 1(k)), and the SafePlay Code lists it among its dangerous tackles. A shoulder charge is where a defender, without attempting to tackle, grab or hold the ball-carrier (or any opposing player) using the arms or hands, makes direct physical contact with the shoulder or upper arm.

The SafePlay Code (up to and including Under 15) also says:

• The tackler's legs cannot be used to trip or throw. An ankle tap is an infringement, which is not to be confused with a genuine attempt to wrap the arms around the legs.
• Adopting a crotch hold is an infringement.
• The ball-carrier cannot be lifted. If a defender lifts one foot or leg, the referee must call "held" immediately. If the tackle continues, or the ball-carrier is placed in a dangerous position, the referee must penalise the offending player.
• It is an offence to sling or swing the ball-carrier using any part of their body, clothing or equipment.
• Any forceful, dangerous or unnecessary contact that involves an unacceptable risk of injury is an offence, regardless of the point of impact.

For all ages under the International Laws, misconduct also includes tripping, kicking or striking, dropping knees first on an opponent on the ground, any dangerous throw, unnecessary pressure or twists (including grapples, crushers and a "chicken wing" hold), and forcefully spearing at the legs of a player in possession. A dangerous throw is a tackle where the player is lifted into a position where the first part of their body likely to hit the ground is the head or neck.

Sources: https://www.playrugbyleague.com/safety/safeplay/ and the International Laws (April 2023), https://www.playrugbyleague.com/media/14513/nrl-international-rules-book-2023_web-version-1.pdf