4 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

What does this card's "Players can't reduce play costs" effect do, exactly?
It negates effects that read, “reduce the play cost by X.” When you or your opponent activates an effect like that, it won't reduce the play cost that needs to be paid. The full play cost of the card needs to be paid.
Ruled Mar 28, 2024
Can "Play this card without paying the cost" effects be activated if this Digimon is in play?
Yes, "Play this card without paying the cost" effects are not effects that reduce the play cost, so they can activate.
Ruled Mar 28, 2024
Can the play cost of a Digimon with the [Royal Knight] be reduced due to the <Your Turn> effect of my opponent's [BT13-007]/[King Drasil_7D6] if this Digimon is in play?
No, they can't be reduced. This card's effect does not target Digimon, so even if your opponent's [BT13-007]/[King Drasil_7D6] is in the breeding area, its effect to reduce play costs can't be activated.
Ruled Mar 28, 2024
When I have this Digimon, can my opponent no longer use [BT14-046]/[Togemon]'s <Your Turn> effect or inherited effect to reduce the play cost of green Tamer cards?
That's correct. Such effects can no longer reduce play costs. This card's effect also prevents play cost-reducing effects from activating when Tamer cards are played.
Ruled Mar 28, 2024
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Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.