Just got reqs, and I'm kind of stuck between votes once again. On the surface, yeah Luna-BM is pretty broken. It has the fundamentals to break through most of the meta and can tech into beating some counters with moves like Hyper Voice or Sub. Moreover, it also exerts a ton of pressure from Team Preview by existing, since giving this thing 1-2 free turns to click CM makes it borderline unkillable. I understand why most people in this thread want it banned, and I kind of do too.
That said, I'm still sort of left thinking that the only reason Luna-BM truly feels so broken is our old friend, Terastallization. With tera, Luna-BM truly goes over the edge from a strong threat to unstoppable. Tera poison in particular prevents it from being worn down via Toxic and also flips Grass and Fighting SE attacks on their head. Removing the water weakness to improve the MU against rain/greninja/waterpon is also huge. Really, it's almost becoming a meme - "Fat mon teras poison and clicks CM to win" could describe Cresselia or Clefable just as well as Luna-BM, even if those 2 aren't quite as broken or potent. And without terastallization, I think it would be much, much more possible to revenge kill Luna-BM. I have some calcs below, more meant as brainstorming ideas than hard evidence:
252 SpA Choice Specs Protean Greninja Grass Knot (120 BP) vs. +1 252 HP / 0 SpD Ursaluna-Bloodmoon: 474-560 (110.2 - 130.2%) -- guaranteed OHKO
252+ Atk Choice Band Rillaboom Wood Hammer vs. 252 HP / 188+ Def Ursaluna-Bloodmoon in Grassy Terrain: 542-638 (126 - 148.3%) -- guaranteed OHKO
252+ SpA Choice Specs Gholdengo Focus Blast vs. +1 252 HP / 0 SpD Ursaluna-Bloodmoon: 416-490 (96.7 - 113.9%) -- 81.3% chance to OHKO
252 Atk Choice Band Great Tusk Close Combat vs. 252 HP / 188+ Def Ursaluna-Bloodmoon: 392-464 (91.1 - 107.9%) -- 50% chance to OHKO
252+ SpA Protosynthesis Walking Wake Surf vs. +1 252 HP / 0 SpD Ursaluna-Bloodmoon: 390-458 (90.6 - 106.5%) -- 37.5% chance to OHKO
252 Atk Choice Band Sneasler Close Combat vs. 252 HP / 188+ Def Ursaluna-Bloodmoon: 390-462 (90.6 - 107.4%) -- 43.8% chance to OHKO
252 SpA Choice Specs Enamorus Grass Knot (120 BP) vs. +1 252 HP / 0 SpD Ursaluna-Bloodmoon: 382-450 (88.8 - 104.6%) -- 31.3% chance to OHKO
Yes, most of the above are choiced super effective attacks that are a roll to KO, although this is without hazards or anything. But I think it goes to show that in a SV OU without tera, there would be a lot more offensive counterplay to contain this thing, and the meta would likely be able to adapt accordingly.
So, just like with most suspect tests this generation, but especially the most recent Kingambit one - do we ban the mon because it is broken as-is, or do we not ban it because the mon itself is mostly blameless? The Kingambit one was a little bit more pointed on the tera issue, with how dynamically tera fairy/flying/dark changes its profile, but I think the two are still comparable. All of this cumulated in Kingambit getting a 55% ban vote, very close to the ban threshold but not enough. This fairly small margin makes me think that this broken on its own/broken with tera dichotomy could be the difference between a ban and DNB result.
I know Smogon isn't a strict judicial system, but I think the Kingambit DNB sort of sets a precedent here. Mons mostly broken because of tera deserve quite a bit of extra leeway. For this reason, if I had to vote right now, I'd probably go DNB, but I'm flexible either way and still open to being convinced in this thread/in OU chat/etc. Beyond Luna-BM, it also opens the question of what exactly could Smogon ban if Luna-BM escapes the axe, and how tiering guidelines should evolve to accommodate generational gimmicks.