As explained by Master Akauma
Tempest Magician:
With a rise in Decks with the ability to quickly accumulate Spell Counters, one can easily wipe out of all of their opponent’s LP, creating an overly consistent One Turn Kill.
True King of All Calamities:
Heavily used monster in “Virtual World” Decks. It has incredible pressure due to its Quick Effect that can seal activated effects and attacks. Since it’s an Extra Deck card, it’s safe to put at 1 without being merciless to those who would like to use it.
Firewall Dragon:
Since it was Limited in January with brand new effects, it has not caused any significant issues, so it is being moved to Semi-Limited. Doing so also helps boost the power of Cyberse Decks.
Performapal Skullcrobat Joker:
A long time Limited card that has generally enabled the flow of plays in “Magician” Decks. By moving it to Semi-Limited, it’s possible new venues and avenues of possibility open up to Pendulum Summon Decks.
Union Hangar:
A Key Card used in Decks whose primary card is “ABC-Dragon Buster”, used for its easy of calling forth other cards, along with having powerful effects. Since “ABC-Dragon Buster” is limited, it’s possible the Deck might see play again with this move?
Altergiest Meluseek & Sky Striker Mecha Modules – Multirole
Due to a major drop in usage by both “Altergeist” and “Sky Striker”, it’s believed this is a fairly acceptable move.