0F 1A and 0F 1B have recently become memory bound verification instructions, as part of MPX. So really, only 0F 1F is likely to be long-term safe to use as a multibyte NOP.

Likewise, after Pentium 4 REP NOP (F3 90) became PAUSE, the spinlock waiting hint instruction, so one needs to be careful about prefixes with the old NOP as well.