Quite often an airfield that has mixed traffic will declare "No deadside" to prevent aircraft from doing a "Standard Overhead Join". This is a locally produced safety instruction and could possibly result in a charge of endangerment if there was an incident involving a breach of this rule however; it is not illegal. The Rules of the Air are enforceable in law and you can be charged with breaching the rule if there is an incident. It is a rule you could break and get away with until something goes wrong and then it will just be another charge added to the list.
Unfortunately, many pilots seem to be unaware of some fundamental Rules of the Air which are there to provide a minimum level of safety and are enforceable in law.
If there is no parachuting then joining from the deadside might not be an issue but all turns within the vicinity of the aerodrome (ATZ) must be in the circuit direction unless cleared by ATC.