It is illegal in the UK, not sure where you are, it will almost defo fail the mot on emissions, not for the missing cat, as this is NOT part of the mot, never has been, people assume it is, but it's not, so long as it passes the emissions for the age of the car then no cat would still pass, it's unlikely to do so though, but some do and a lot just decat for the year put cat back on for mot swap over afterwards again.
tabetha