If a creature loses all abilities, does it lose its triggered abilities already on the stack?

When a triggered ability triggers, it is placed on the stack as an independent object, separate from the permanent that generated it. At that point, the ability exists on its own and is no longer dependent on the source having that ability (CR 112.7a).

If the creature subsequently loses all abilities — due to an effect like Sudden Spoiling or Turn to Frog — that loss of abilities does not reach back and remove triggered abilities that are already on the stack. Those abilities will still resolve normally (CR 112.7a, 603.3).

This is because once an ability is on the stack, it is treated as an independent spell or ability object. Losing the ability from the source has no retroactive effect on copies already waiting to resolve (CR 112.7).

Example: Your Grave Titan attacks, triggering its 'create two 2/2 Zombie tokens' ability. Before that trigger resolves, your opponent casts Sudden Spoiling on Grave Titan, making it lose all abilities. The trigger is still on the stack and resolves normally — you still get the Zombie tokens.

Note: If an ability says 'At the beginning of your upkeep, if [source] has this ability, do X' that's a different matter involving a condition check at resolution — but the ability being on the stack itself is never removed by the source losing abilities.