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The 0F 18 has been known for a long time, and they are not "true NOPs" but PREFETCH instructions:

http://x86.renejeschke.de/html/file_module_x86_id_252.html

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.

Although PAUSE only has a performance impact, it was part of its design (so you need only one binary for your spinlock, and it executes correctly even on processor who don't know anything about PAUSE)

I think they were introduced in the P6 as NOPs, and some of them (but not all of them!) later turned into PREFETCH instructions.

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