It's my understanding that the IMAX presentation is 1.90:1 vis a vis shooting Alexa open gate 4:3, therefore the top and bottom are protected for IMAX. I think most multiplex IMAX screens are formatted for 1.90 though I can't be sure of this, I doubt that there are scope IMAX dcp's unless otherwise noted.
I think the 15/70mm IMAX screens are 1.43 in size but can exhibit multiple ratios. Afterall they have previews that vary from 1.85, to 2.40, to IMAX 1.90. But I'd figure most digital IMAX and multiplex IMAX screens are 1.90. And all versions of BR 2049 for IMAX *should* be 1.90 as that's the AR for digital IMAX.
Please keep in mind this is just a guess from personal experience of just a few IMAX theaters in the Los Angeles area over my lifetime.
I saw it both in a normal theater 2.40 and IMAX Laser 1.90 and both were phenomenal presentations. I saw the IMAX version at Universal Citywalk in LA which has a 4K laser projection, and the contrast was incredible. Also was sitting center seat-- middle row, so can't comment on how other angles were affected.
I would be more interested in the projection resolution than aspect ratio, unlike Dunkirk where the majority of the IMAX footage covers it's 1.43 15 perfr 70mm negative area, I think all IMAX theaters will be able to show BR 2049 in 1.90. I'd rather see it in 2.40 4K than old school Xenon IMAX 2K. But that's just my opinion.
Mr. Deakin's said his preference was for 2D 2.40 widescreen, so there's that to keep in mind also.
Either way you see it, it's a stellar film with some of the best photography I've seen. I doubt you can go wrong. Hope this was somewhat helpful, hopefully someone more knowledgeable than myself will correct me if I'm wrong about anything.