Anything that is able to win roughly half of the games could do that eventually, which isn’t amazing, to be honest.
It’s probably popular due to the free deck and/or because it’s cool, but that’s about it. About the latter point — less so since it no longer disrupts P2W Swaggering Bozo Reno decks (still does disrupt the singleton Paladin deck from Whizbang… which figures, I suppose).
It’s still too slow most of the time.
I’ve found that even a ‘rainbow’ (BUF) DK would beat a ‘pure’ plague DK most of the time, even though the former runs only a lesser plague package. As for the rest of match-ups…
First, I suppose, it’s worth mentioning that there used to be roughly two variants of plague DK: UUF with the excavation package, which was/is probably the most common one, and a tankier BUU. The former is a bit greedy, but very slow and vulnerable, especially to aggro. Moreover, I don’t even know why people considered it so good vs Odyn Warrior, which was notorious for drawing a lot of cards — nothing prevented them from brushing off some plague damage and just doing an OTK manoeuvre, once their Odyn was up; in fact, I’ve found an ‘RGB’ DK to be much more effective with a lesser plague package, more as a bonus and a finisher, while you could hold their face with freezimg weapons, as Helya slowly did her work (so satisfying, btw). But anyway, most of DK’s ‘tankiness’ comes from at least one Blood rune (although BUU still cannot afford excellent Quartzite Crushers in the deck), I experimented a bit with it (see also this topic, although my approach was a bit different) — it was more resistant to aggro, but suffered from insufficient card generation (something the excavation package helps remedy), which, combined with ineffective cards such as the Staff of the Primus or Distressed Kvaldir (you really don’t want cards that just do a little something in modern HS; it’s mostly about cards that do something major and/or generate more cards — or something along those lines), would result in the deck’s running out of steam. Ironically, sometimes it would lose even to singleton Warriors for this reason, despite disrupting their stuff with plagues. Someone had reportedly achieved more success with this deck by adding Reno to it [1], but that’s not relevant anymore anyway. Another curious point: I did win some games vs those annoying Shamans with this approach, though, since they would draw a lot, and more plagues were sometimes capable of disrupting their game plan — perhaps this was a match-up when plagues alone (well, almost: healing etc from the Blood rune would still help to recover from not-quite-OTKs sometimes) were most effective.
[1] https://www.hearthstonetopdecks.com/decks/unholy-blood-plague-death-knight-22-legend-unknown-whizbangs-workshop/
Anyway, as I was trying to say, plagues were mostly a bonus, including disrupting singleton decks — well, not anymore, and a way to eventually win longer endgames if your opponents drew a lot of cards — they didn’t really solve everything on their own (you’ll probably realise they are vitually nothing in comparison with post-patch T.N.T., which works the same way now). By the way, that’s why ‘RGB’ DKs were much more successful — some plagues and notably Helya were still a bonus, while they traded the rest for much better and efficient cards overall.
PS Some minor updates and edits made, besides, I’ll add a little comment: the past tense is used, since it’s no longer relevant anymore anyway in light of Warrior’s virtually absolute dominance after the recent changes (see above or something like this, for instance). Sure, some people are playing decks like spam Paladin or excavate Warlock (which is reportedly only a thing due to some bug, but I dunno the details, tbh) or just trying to play whatever they want for the fun of it, but frankly, it seems that, in a competitive sense, most other decks and classes are, as said, just irrelevant against the backdrop of almigthy P2W Wallet Warrior Agency™ (alreadly mentioned Reno, with bonus points for the irony of it, earlier), thus the discussion about which type of DK is better seems rather abstract to me. Budget or subjective fun is another matter, but that’s about it.