Is Stealth actually relevant?

Hunter also ignores Concealment, but there honestly not that many models that have it...

Part of it, I think is ease of access to the anti-Stealth measures as part of themes (or even Stealth itself). Some combinations can deal with Stealth real easy, others can't... it's when the Stealth is combined with something else that can really be a bugger... namely high ARM (Occultation on the Sacral Vault against a single-wound heavy infantry list can be really obnoxious).

I've been toying for a while with an Ossyan list... his whole shtick is blasting people off the table on his feat, but there is only one way for his list's guns to deal with Stealth'd high ARM, and only one unit can do anything about it. *shrug* But I can't cast judgement, since I've never actually played the list (my need to have a list pairing that can deal with recursion, cloud wall spam, stealth spam, boxes, ARM skew, anti-magic tech, N3mo/Hal3y, the mirror, B2-Bones, Coven-DH etc. means that Ossyan is a bit crap).

Nobody would take a shooting list that can't deal with Stealth, because then you lose the first time you encounter significant stealth. Stealth invalidates itself by being too effective and forcing an arms race. Yes, stealth is not a great defensive mechanic. Why not? Because ranged attacks that cannot circumvent stealth somehow are not worth taking (because stealth). It's this hierarchy of (ignores stealth) > (stealth) > (doesn't ignore stealth) with each tier somewhat invalidating the lower.

Concealment is in my mind better, because it's not binary. I don't lose nearly as hard if I take a ranged unit that cannot ignore concealment - hitting just gets a little tougher. I have options (CRA to hit but kill fewer, gamble on more shots, def debuffs etc). Here you might take a unit that cannot ignore concealment, knowing that they can still contribute. If it was stealth, that unit would not contribute. Even if there were more abilities to ignore concealment, you might not see them because the penalty for not having them is not as harsh. With stealth: you're effectively being told "Hey, you can take this unit and shoot at what you want 60% of the time. Or, pay 2 more points and shoot 95% of the time" When you're taking a shooting unit, why the hell wouldn't you? The penalty for not having it is too harsh.

I was exaggerating for dramatic effect, you're not going to immediately lose without recourse the first time you encounter stealth. It depends on how much and what of the opponent's army has stealth. If you have a gunline (or really any ranged unit) you want them to contribute against their designated target from range. If they can't, they can still contribute to the game (contesting, engaging, screening) but almost certainly that is not an efficient use of points. Better than nothing, but not what you want them doing.

Consider a gunline of warjacks and infantry. In most cases, such a gunline will have high POW, low volume shots on the 'jacks, and low POW, high volume shots on the infantry.

If that gunline encounters Shadow Pack, then the warjacks need to be able to see stealth. The low POW of the infantry is unlikely to significantly damage many warjacks/warbeasts.
If that gunline encounters lots of Stealth infantry, then the infantry need to ignore stealth because the warjacks can't kill enough.
If that gunline encounters a mix of stealth heavies and infantry, both components need to be able to see stealth, or the ones that can't won't contribute effectively.

So if you can give one model/unit the ability to ignore stealth, or remove stealth from one target, that helps. If the opponent has lots of stealth, then it is probably not enough.

Obviously you have lots of control over what you encounter because you have a second list. You can play a gunline that cannot ignore stealth with a single model, and only play it against opponents who don't have any stealth models in either list. But, the more you don't want to see the more you limit yourself.

Honestly it's not like gunlines are very effective anyway, with scenarios and some of the ludicrous threat ranges possible. But you can reduce the discussion down to units instead of whole armies and not change much. If you take a ranged unit with low POW attacks, and the opponent has two units that both have stealth, what value does that ranged unit produce? The ability to ignore stealth makes a unit more versatile, which is necessary when you don't have full knowledge over what you will encounter. Maybe the versatility is produced by taking two specialist units, that attempt to cover each other's weaknesses. That's not how I prefer to play, because I find it unlikely that when encountering such a weakness, the one unit that is effective can do the work of two. And that's what it would have to do against stealth. Some other defensive mechanisms merely result in reduced effectiveness, not eliminated effectiveness.

That was pretty wordy, hopefully it makes some sense.

Sadly, gun lines are still dominating my meta, specifically that of Retribution and Khador (Vlad1), and to a lesser extent Circle with the Wyrds. It's annoying.

Stealth is generally fine. The problem arises when it's both possible to make a list that is entirely stealth and at the same time make a shooting list that totally ignores stealth.

IMO, stealth heavy lists should have non-shooting related weaknesses so that a non-shooty list can deal with them. And at the same time, it should be impossible to make a gunline that totally ignores stealth, because many of these stealth lists have their only real weakness being stealth ignoring shooting.