Non, ajouter ou perdre une capacité ça serait "has flash" et "loses flash" (cf. exemple ci-après), dans ton exemple ce n'est pas ça.
Example: Two effects are affecting the same creature: one from an Aura that says "Enchanted creature has flying" and one from an Aura that says "Enchanted creature loses flying." Neither of these depends on the other, since nothing changes what they affect or what they're doing to it. Applying them in timestamp order means the one that was generated last "wins." The same process would be followed, and the same result reached, if either of the effects had a duration (such as "Target creature loses flying until end of turn") or came from a non-Aura source (such as "All creatures lose flying").
PS: Si chaque joueur contrôle
Teferi, Mage of Zhalfir, alors chaque joueur ne peut jouer des créatures que quand il pourrait jouer un rituel. Ton propre Teferi donne le flash, mais le Teferi adverse ne fait pas perdre le flash, il rajoute une restriction.
AVATAR