Does an effect that removes all abilities from a creature also remove triggered abilities that have already triggered?

When a triggered ability triggers, it is placed on the stack as an independent object. At that point, it exists separately from the permanent that generated it. Removing all abilities from the source creature afterward has no effect on abilities already on the stack.

This is governed by CR 112.7a, which states that once an ability is on the stack, it exists independently of its source. The ability will resolve normally even if the source permanent loses that ability or leaves the battlefield entirely before the ability resolves.

However, if the effect that removes all abilities is applied before the trigger condition is met, then the ability no longer exists on the permanent and cannot trigger in the first place. The timing matters: abilities are removed only from the permanent going forward, not retroactively from the stack.

Concrete example: Your opponent controls a Siege-Gang Commander with a death trigger. You cast Sudden Spoiling, removing all abilities from their creatures. If one of their Goblin tokens was already destroyed before Sudden Spoiling resolved — and that death trigger is already on the stack — that trigger still resolves and deals its damage. But if the Goblin dies after Sudden Spoiling resolves, the token no longer has the death trigger and nothing goes on the stack.

In summary: effects that strip abilities only affect the permanent in the present and future (CR 611.3). They cannot reach back and remove triggered abilities that have already been placed on the stack (CR 112.7a).