Is a BB spindle sleeve required?
It's not required for the BB to function or the bearings to be located correctly or loaded correctly. It is only a shield against contamination, particularly from water that gets into the seattube at the seatpost opening. That said they're actually pretty important parts that do a lot to make the life of the BB what it should be.

I have no idea why yours might be missing. My guess would be a mistake when assembling or servicing the bike. Make sure it's not still in the BB shell. They pop on and off the cups easily so it could have gotten stuck in their. Sometimes you have to loosen the BB cable guide mounting screw for the BB sleeve to pass through.