just check the cast list. Robert Downey Jr currently not there.

Tony Stark and Iron Man are also currently absent.

I don't think its too hard to find an actor with very good voice impressions to RDJ

Evans and Downey are unlikely to return despite their characters likely appearing.

Also, the 23 episodes thing hasn't been confirmed and is unlikely. Feige made a vague statement that was just interpreted by some as 23 episodes.

Considering they brought back the other actors, I doubt they would recast someone’s voice just for this show. I don’t think they will have him in here, but he could be a surprise saved for later.

I think since we know Steve Rogers is in this show but there has been no discussion of Chris Evans returning, and Evans has said he's done with the role, that they are almost certainly getting someone to do an Evans impression for Steve, so if they're willing to do that I reckon there's a very good chance they'll get someone to do a Downey impression for Tony.

I wouldn't mind having all actors recast for What If...?. Its voice-acting we are talking about... I don't think we will need a real Chadwick Boseman, for example, to make the impression of Black Panther.