Nimble is basically a rewrite of skulk from SOI, which did not last. From the same set was Odric, who had a “share the words” ability written out in full. Oh hey, skulk is on that list!

Descend is not a keyword ability. The only ones on that bat are flying and lifelink. Rules pedantry below.

A keyword ability is one where the term has a specific rules meaning on its own. You can tell because it isn’t written as part of a sentence, like flying, ward, and offspring. Note how there’s never a period at the end of the word; just the word (plus some other element like a cost) on its own represents a whole ability. In Arena, these are the words that are bolded.

A keyword action is a word with a specific rules meaning as an instruction, so it actually is part of a sentence. Surveil, mill, open an attraction, and forage are all actions that a sentence of rules text tells you to do. Cast, attach, and destroy also count.

An ability word is an italicized word that highlights a common mechanical theme, but has no rules meaning at all (that’s why it’s in italics, like reminder text). Examples are landfall, threshold, and valiant.

Descend 4, descend 8, and fathomless descent are all ability words, not keyword abilities. Interestingly, “if you descended this turn” is none of these. That phrase has a dedicated section in the comprehensive rules (700.11) that defines how it works. This section also defines historic, outlaw, committing a crime, and expend.