W16/W17 handled inconsistently for Pioneer and Explorer legality, not clear what's correct

[scarletcs]

A problem's come to our attention with the following sets. They are being handled inconsistently for Pioneer and Explorer legality.

• Welcome Deck 2016 (W16)
• Welcome Deck 2017 (W17)

This impacts at least the following cards:

• Bloodhunter Bat
• Raise Dead
• Stormfront Pegasus
• Soul of the Harvest

Background: At the time, the Welcome Decks were standard-legal for the sets they were released alongside so that new players could bring them in to play at their FLGS without getting in trouble. (Wouldn't make for a very good welcome if they did!) W16 was released simultaneously with SOI, W17 was released just before AKH.

The following search identifies cards that match the following criteria: they are in W16 or W17, and they are not present in any other standard-legal set since RTR onwards. Meaning, W16/W17 was their only standard-legal presence within Pioneer's window.

https://scryfall.com/search?q=%28in%3Aw16+or+in%3Aw17%29+date%E2%89%A5rtr+-in%3Am14+-in%3Am15+-in%3Am19+-in%3Am20+-in%3Am21+-in%3Aemn+-in%3Aktk+-in%3Artr&unique=cards&as=grid&order=name (13 cards)

Three of them are also present on Arena: Bloodhunter, Raise Dead, and Soul of the Harvest.

https://scryfall.com/search?q=game%3Aarena+%28in%3Aw16+or+in%3Aw17%29+date%E2%89%A5rtr+-in%3Am14+-in%3Am15+-in%3Am19+-in%3Am20+-in%3Am21+-in%3Aemn+-in%3Aktk+-in%3Artr&unique=cards&as=grid&order=name (3 cards)

Each card present on Arena that is legal in Pioneer is expected to be also legal in Explorer, because Explorer is expected to be all cards on Arena that are legal in Pioneer.

Now, let's look at how Gatherer, MTGO, Arena, and the tournament rules evaluate these cards' legality in Pioneer and Explorer:

• Gatherer considers each and every one of them to be Pioneer legal.
• Gatherer also considers the three present on Arena to be Explorer legal.
• Arena, however, does not consider those three to be Explorer legal. (If they were Pioneer legal, they should have been grandfathered into Explorer.)
• MTGO considers all three of those Arena cards to be Pioneer legal. (We didn't check all 13, but I expect they would show up the same way.)
• Pioneer has an exhaustive list of sets that it considers legal for Pioneer play, both on the website and in the Magic tournament rules (section 6.7, top of page 35). This list does not include Welcome Decks 2016 or 2017.

So:

• Pioneer legality: Gatherer says they should be Pioneer legal and MTGO agrees. However, the MTR does not apear to include them, so it seems like it might consider them not legal.
• Explorer legality: If they were Pioneer legal, they probably ought to be Explorer legal too. Gatherer says they should be Explorer legal. Arena, the actual game client used for Explorer, considers them not not legal.

It's unclear to me which sources in this situation are correct. Are the two Welcome Decks legal for Pioneer or not? Should they also be legal for Explorer or not?

---

Above findings but as a table, evaluating whether each source considers these cards Pioneer/Explorer legal. -- means the source does not have an explicit stance.

Source	W16/W17 are Pioneer legal?	W16/W17 are Explorer legal?
Gatherer	Yes	Yes
Arena	--	No
MTGO	Yes	--
MTR	No(?)	--

[scarletcs]

Related: Wrath of God is presently recorded as banned in explorer, but it shouldn't be banned in explorer because it shouldn't be legal to begin with. (It's not a pioneer card.)