It has no upfiring speakers, so even if it could decode Atmos, it can't play it properly.

And the same with the Amp….

At least until such time as Sonos decides to come out with special “Atmos” speakers, the Arc will be the only device that can deal with Atmos. 

The Playbase and Playbar have been discontinued. This means that they are no longer being manufactured but does not mean that they won’t be upgraded to S2.

However, on that basis I believe it can be surely assumed that Sonos will not commit resources to any major upgrades of the basic functionality of the two products. An Atmos implementation would likely require and updated licensing deal for these products and would also need to be accompanied by either some dedicated Sonos-Atmos speakers or a way to pair them with two Sonos Amps: One to drive rear-speakers and one to drive in-ceilings. 
 

Finally, and that I guess is the most obvious reason why Atmos will never come to the Playbar or Playbase: both speakers use an optical connection rather than HDMI as on the more recent products. The optical cable simply will not carry an Atmos-signal. The same applies for Dolby Digital Plus. 
 

For the Amp and Beam? Highly unlikely but not 100% impossible. 

You cannot send Atmos over optical, so the Playbar/Playbase will never be able to decode it, much less play it.