I think all of the Splatoon 2 bundles come with 2.1.0. But something to bare in mind if you're buying off eBay is that sellers might switch the tablet with a different one inside the box, so its always best to ask.
For example, I've seen a seller list a Switch tablet with 2.1.0 at a premium price, then they've listed the rest of the box contents (joycons, dock, etc) in a separate listing, and then also listed the whole Splatoon 2 Switch bundle in a third listing at a premium price. When enquiring about the 2.1.0 tablet, I was told it was from a Splatoon 2 bundle.
Call me a cynic but I couldnt help thinking that this seller has taken the 2.1.0 tablet out of the Splatoon bundle to sell on its own for a higher price, and then stuck a higher FW Switch tablet into the Splatoon bundle box and then listed that also at a high price. Baring in mind that the 2.1.0 listing mentions its use with homebrew, while there's no mention of the firmware version in the Splatoon bundle listing.
I'd imagine you can see what's going on here...