...the legality of a forward pistol grip on a AR. Is it no longer legal to have one on a rifle?

I'm not aware of any limitation in WA for an already-owned, legal "assault weapon."

You can no longer buy, sell, or build an AR-15, of course, regardless of the grip on it.

However (and this is hazy)
can you have a forward grip on an AR 'pistol'?
IIRC (and this may be wrong) that is not allowed.

Not a vertical grip, no. An angled grip . . . well, those are generally regarded to be okay, though I don't think there's clear ATF guidance on it.

If your AR pistol has an overall length of more than 26 inches*, then you can have a vertical forward grip without the firearm falling under NFA classification. It just ceases to be a "pistol" and instead is simply a "firearm" (Title I under GCA).

*Measured from the end of the muzzle (if it has a removable muzzle device, then measure to the end of the barrel's muzzle threads; if it has a pinned/welded muzzle device, measure to the end of that device) to the rear most part of the firearm [if it has an arm brace, measure to the end of that brace (if it's an adjustable brace, measure it in the fully extended position); if it just has a buffer tube, measure to the end of the tube].

Agreed, I should have included that distinction in my post.

Thanks for the clarification. I don't own any 'pistols' under discussion so I am not up on the details.

Didn't the most recent ATF position exclude braces from the OAL measurement? I think that's where we're at again with the injunction on their "braces are stocks" rule.

You guys......

I am just going off of logic...if braces are stocks, and stocks are measured in the fully extended position, then so are braces. But it probably does depend upon what ATF had for breakfast today.