Exclusively?
Rain of Steel - the only thing the Effect does is damage. Flames of Phlegothos - the only thing the Effect does is damage. There are plenty of examples...
You're right, of course, it's not exclusive. I spoke too hastilly and with only a cursory glance at the powers.
We would agree though that whether because of the fact that they are effects or because they cause damage to the enemy, any immunities would be in full effect in either case. Now that you have splintered this part of my argument these examples are not in contension.
Strictly, in the case of Poison Immunity, it talks about poison damage, or any other ill effect.
Note the use of the word 'other'. The implication is that poison damage is an ill effect; otherwise, it would be phrased as "poison damage, or any ill effect". If poison damage were not an ill effect, the word "other" isn't appropriate.
Point taken, but the implication of the wording is
somewhat less definitive than your excellent example. More difinitive would be if they had said something like "any ill effects, including damage" but that's just wishful thinking and not additive to any conclusion here.
It may also be helpful to quote the full definition from MM since some parts include the notorious "other" and some don't.
MM 282 "Immune: The monster has immunity to the stated kind of damage or effect. For example, a monster with "immune poison" never takes poison damage and can't suffer any other ill effects from a poison attack."
First, before the example there is no use of the word "other". "Damage" and "effects" are
suggested to be different. If damage was an effect you might as well say something like , "I know how to fix Honda's and cars".
But then they go and confuse things with the example. But still separate damage from "other effects". Why? Perhaps you're right, it is an effect, but even the example suggests that it needs to be treated separately.
Lastly, if immunities were as comprehensive as you say, wouldn't it just be simpler to say that "Monster with immunity are completely unaffected by powers with the stated keyword"?
I think the subtlety of denoting "damage and (other) effects" was deliberate and allows powers with multiple keywords to be at least partially useful in those situations. If you're right, (and I'm leaning that way as I write) then I think the "other" is there to note that it's effects "other than damage" that it's talking about.
I think I can get onboard with your position that damage is an effect of a power, but I don't agree that a power that does 3 seperate damage types (like Rain of Spines) can be completely neutralized with only 1 immunity.