Is this only for recently bought switches? I bought one in Japan in April this year at Toys r Us and it came with 4.1.0. I already updated it to 5.1.0. I haven't tried RCM tho.
We really have no idea when these appeared (could be days or weeks, and we just haven't had any one report it until now), and they are part of a mixed batch. But April is probably safe.
There's not that many people buying a Switch to hack it, so its very hard to know exactly when these things started showing up in user hands.