The BT15 landscape
BT15 has stabilised into just 3 top-tier decks.
At the very top we see Apocalymon cementing its position as the undisputed Tier-0 deck, while Anubismon and Metalgarurumon are both very strong Tier-1 OTK decks.
Every other deck other than these 3 has seemingly become dust.
Apocalymon is both Meta and Anti-Meta
In the EX5 that came before BT15, we saw the extremely potent Anubismon/Mervamon deck. With minimal setup, Anubismon can very efficiently flood the board with Mervamon and minions that can either rush, block, or board-control. Anubis/Merva is a relatively flexible toolbox deck that has just about everything needed to counter every other deck. See DDDTCG/Robin's write-up about this deck here!
Apocalymon serves as a good counter to the Anubis/Merva deck. It trivially punishes Anubis's blatant milling by speeding up deck-outs, and with correct tech card (Magnadramon) it slows down Anubis/Merva just enough for deck-out. However do note that currently Apocaly and Anubis can still be quite evenly matched.
In fact, Apocalymon's milling can be so exorbitant that it becomes highly plausible to simply mill away your opponent's anti-Apocaly tech cards (such as Merciful mode), while the more trivial "cannot reduce play cost" rookie effects can simply be met by a simple Death Claw. To give context on this level of milling, each Apocalymon would cost only 3 to play and mills 8, so a deck size of 40 cards (discounting 10 cards in security and starting hand) takes 2 Apocalymons just 2 turns to deck-out.
Bandai's recent October 2023 banlist saw restrictions on the milling Gabumons and Eyesmon, largely in attempt to nerf Apocalymon. However, this banlist merely steered players from the Eyesmon build to a Garurumon-X build. And ever since Apocalymon has proven its consistency, players have started experimenting (with success) different tech cards for the Apocaly engine, such as Beelstar/Tidal and Ukkomon.
Bandai even went as far as promoting Apocalymon's weakness on their weekly "Digimon Card Battle" episodic on Youtube, which showcased Aoyama's Apocalymon deck suffering consecutive defeats due to slow milling. However, with the mulligan rule and BO3 format, experienced players can often fix a bad starting hand, and given that Bandai almost never bans cards from their most recent sets (for obvious reasons) it would seem that Apocalymon will be staying at the top for at least a couple more months.
In BT15, almost every deck has to be made 'Apocaly-proof', and there are several ways that players are going about.
Most contending Tier-1 decks are built for OTK to take advantage of Apocalymon's lack of offense. In a game of speed, if Anubis/Merva or Metalgarurumon manages to OTK before Apocalymon mills out Magnadramon/Metallicdramon or for more Apocalymons then the game can be pretty much secured.
While other deck that uses 'Omegamon Merciful Mode' may have a chance at beating Apocaly at its game, they may not fare as consistently if the deck was not built to 'draw Merciful' and winning quickly. Furthermore, a lucky Apocalymon player will use a Craniamon for protection, and every turn an Apocalymon stays on board is a massive horror story for the opponent.
OTK decks such as Loogamon relies on blatant early game mills to setup the hand and trash, properties punishable by Apocalymon. Since the Loogarmon line does not have protection, the OTK mechanic can be easily thwarted by an Apocalymon inheriting both Blocker and Retaliation. Similar can be said about Bloomlord decks.
Sec Con decks have also seen a sudden disappearance due to its complete ineffectiveness against Apocalymon. Aside from being too slow, recovery of security is a completely meaningless against a deck designed to deck-out. Other variants such as Agumon/Hybrid are also less potent since value cannot be generated from security hits.
Apocalymon has managed to cripple every other deck, while the only 2 other surviving decks try to find their inflated self-worth amidst a barren land. Done were days of healthy deck-building, variety, and innovation. For weeks all we see is a purple trifecta, and the consolations being minor innovations that sets each deck every slightly apart.
BT15 is truly an Apocalyptic Dune.