If a creature loses all abilities, does it lose triggered abilities that have already triggered?

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

This is governed by CR 112.7a, which states that once an ability is on the stack, it exists independently of its source. The source's current characteristics are irrelevant to whether the triggered ability on the stack resolves — only certain specific effects (like 'exile it' targeting the stack object) can remove it.

CR 603.2 explains when triggered abilities trigger: they go on the stack the next time a player would receive priority. Once that happens, even if the permanent immediately loses all its abilities (say, via a spell or static effect), the trigger already on the stack is unaffected.

Concrete example: Your Ravenous Chupacabra enters the battlefield, triggering its "destroy target creature an opponent controls" ability. In response, your opponent casts Turn to Frog on it, making it lose all abilities. The triggered destroy ability is already on the stack and will still resolve normally when it gets priority — your opponent's creature still gets destroyed.

The only exceptions would be effects that specifically counter abilities on the stack (like Stifle, CR 117.5) or that specifically prevent triggered abilities from being put on the stack in the first place. Simply removing abilities from a permanent does not reach back to the stack.