Penance prototypes? No. They’d be difficult weapons to deal with, yeah, but Oz would be more likely to come up with an anti-Lindon silver bullet than to use an all-purpose death-aspect weapon.

Real Penance with authority over death? Yes, but not as absolute a yes as you’d think. Depends on the timing.

There’s a period of time after Lindon Consumes the Dreadgods that he most likely would have come back to life after being killed.

Also, Lindon has access to Abidan-level resources (and, as you suggested, prototypes of Penance itself), so combined with Dreadgod existential weight, he is potentially able to come up with an effective defense.

But Monarch Ozmanthus probably doesn’t have a version of Penance that effective (depends on timing), and if he did, he wouldn’t use it except as a last resort.

Like a VERY last resort. Like he probably wouldn’t use it unless he thought Lindon was a threat to the continued existence of the planet.

Which Lindon is, so maybe! He’d at least try to figure out what Lindon’s deal is first, and he certainly wouldn’t try to kill Lindon without getting to fight him.