I'll go out on a limb (admittedly, a pretty safe one) and say that yes, Arthas NEEDS to be in Shadowlands. Blizzard is aware that he is, without exception, the most well-known character in the franchise. They are also keenly aware that information gets around very quickly when it comes to datamining the Beta, and so I think Blizzard is simply avoiding Arthas initially, to have a greater build-up to his eventual reveal.
I would expect to see him hinted at in whatever the Tier 2 raid is for Shadowlands, and his actual return confirmed in Tier 3 (or even Tier 4, if they stretch Shadowlands out that long). Whatever the final patch for Shadowlands is, that's when we'll ACTUALLY see Arthas, I think. Would be nice to also see Ner'zhul, though!