I would prefer "is Frozen". Having the ability with that name sounds like it would freeze others.

Compare "is goaded".

Interesting point. brings up the distinction between ability and state.

Per your recommendation, frozen is now a state, as a consequence of the 'freeze' keyword action. A frozen permanent ceases to be frozen the moment it untaps.

Furthermore, Eternally Frozen is a special case of frozen. A permanent that is Eternally Frozen always counts as frozen, regardless of whether the permanent is tapped or untapped.

Huh, yeah - I can see how that might be common enough to keyword.

One issue that probably came up in alpha for this set - "tap" is three letters long. "freeze" is five. It seems stupid, but given how many times the word appears, it probably matters. After all, they very very quickly moved to the symbol for even more space saving.

Maybe in this world, the word 'freeze' eventually goes away completely and we get a {f} symbol? :)

Frozen / eternally frozen reverted to keywords for simplicity to code in Forge.

Frozen is basically a replacement effect. Eternally frozen a static ability.

Something "is frozen" if it has the keyword frozen and/or eternally frozen. (Note distinction between "is frozen" and "has frozen".)