I just checked the files and they never get phased out.
If your game stopped seeing them spawn that was just pure chance that all your recent missions didn't contain them.
The Muton Centurion has a chance of spawning in any alien-permitting group from force level 8 onwards but is never guaranteed to appear anywhere.

Originally posted by Melancholia:

I just checked the files and they never get phased out.
If your game stopped seeing them spawn that was just pure chance that all your recent missions didn't contain them.
The Muton Centurion has a chance of spawning in any alien-permitting group from force level 8 onwards but is never guaranteed to appear anywhere.

Well it is just not me - look at the comments. A lot of people apparently never see them anymore.

You sure everything is as it should?

I will say that I did just see I think 2 Centurions in a Commander campaign.

To be Honest they should be an upgrade to Mutons not another type of Mutons... I'll explain my point: They could let the Mutons into the campaign for a while and then introduce The Centurions as an upgrade of them later on. Like what happen with ADVENT forces, they all get replaced by a better version of themselves. E.g. The Advent trooper becomes an Advanced trooper and later upgrades even more to an Elite trooper for balance purpuses between them and your squad being stronger with time. It would make much more sense that in the late game Centurions replace Mutons. Which it's not the case, Centurions just comes by around mid games and are gone like 2 or 3 months later...

have to agree, never saw them again in late campaign (all missions very difficult), but i did see normal mutons). Also, final mission, 9 normal mutons, no centurion.

Maybe the phasing out is due to higher tiers of enemies getting spawn priority? Once Andromedons and Sectopods start appearing, the mid tier units phases out almost completely. Maybe there's a way to include Centurion as escorts?

I have no idea how XCOM 2 alien spawn codes work, so this is just a guess based on what I've read elsewhere. =)

Well, just think of them as... Muton Mercenaries instead, hired on after the cloned Mutons fail to eliminate XCOM, and then after suffering enough losses, they're like, "♥♥♥♥ it we out."