Can I react to shens “when I hold with one other unit I score” or any passive type abilities

Yes for Shen's hold ability — no for genuine passive abilities.

Shen, Leader of the Kinkou Order reads "[Shield] When I hold, if there is exactly one other unit you control here, you score 1 point." The "when I hold" clause makes this a Hold Effect, i.e. a triggered ability, so it's put on the chain as a pending item and every player gets a Reaction window before it resolves (Rule 383.4.d). What you can't react to is the hold/score event that caused it.

One nuance: the "exactly one other unit you control here" clause sits immediately after the trigger condition, so it's part of the condition, not the effect (Rule 383.2.a.1) — it's checked when the trigger fires, and killing that other unit in response won't stop the point. Once the trigger is on the chain it resolves.

Passive abilities are different: they're statements of fact with no "when/at" condition (Shield is a passive, for example), they never use the chain, and there is no priority window in which to react to them — they just continuously apply.