Which Traitor Legions can still create new Marines?

Question

One of the downside of the embracing Chaos and its gifts are mutations, that would also affect the progenoid glands, necessary to create new Marines. In some cases (Rubric Marines), such organ completely ceases to exist.

So which of the Traitor Legions are still able to create new Marines "from scratch", not from converting loyalist or stealing geneseed from loyalist chapters?

I know that Iron Warriors definitely still do produce new Marines and the number of Thousand Sons is constant (they cannot be truly killed but also can't reproduce). Word Bearers are apparently surprisingly mutation-free, but what about such "gifted" legions like Death Guard, Emperor's Children or World Eaters?

Answer 1

Death Guard

From Lords of Silence novel:

It’s odd, how a Legion works. Many of the Lords of Silence are Barbarans, taken from the gene pool of that mist-wreathed hell planet. A slim majority, though, are not. Most of the non-Barbarans were created in the Eye from stolen gene-seed, implanted by the Surgeons into screaming infants wrenched from feral Imperial planets, and thus have no connection with the forgotten home world. Others, like Dragan, are turncoats and renegades, refugees from distant Imperial Chapters and warbands. Somehow, though, over time, they all adopt the taciturn habits of Mortarion’s own.

With no words it is mentioned that there are new Lords of Silence from their own gene-seed. It can therefore be assumed that there are no more new Death Guard Space Marines.

Emperors Children

Like all the Traitor Legions, however, the Emperor's Children's gene-seed is now so corrupted and riddled with Chaos mutations that it is functionally useless for creating new Astartes. As such, the Emperor's Children, if they could pull themselves away from their self-indulgences long enough to care, must steal pure gene-seed stores from the Loyalist Space Marine Chapters when the opportunity presents itself if they seek to increase their numbers.

From the Fandom article on Emperors Children

World Eaters

Fabrikus was a brilliant Apothecary in the World Eaters Legion during The Great Crusade and served with its First Company, where he gained distinction as a warrior and as a surgeon.

...

Fabrikus would eventually be charged by the dark masters that commanded him with creating more Chaos Space Marines for their Long War against the Imperium, and he spent several centuries experimentally implanting various species with the Gene-seed available to him in the Eye of Terror. He met with constant failure, however, as the Gene-seed refused to take, or else produced unhelpful mutations in his subjects. Though he would never say this to his masters, Fabrikus came to believe that the Warp caused potency problems with the Gene-seed from their warriors and would not produce the results expected of him. In order to rectify the problem, Fabrikus has decided to hunt down and harvest the Gene-seed from Space Marines loyal to the Imperium, whose belief in the Emperor ensures their Gene-seeds are free from the taint of Chaos. With the Gene-seed taken from Space Marines, Fabrikus is confident he will have the material needed to create a new type of warrior, completely loyal to the Chaos Gods and unstoppable in battle.

From the Lexicanum article on Fabrikus

With that, it can be assumed that the World Eaters are not capable of creating any more Marines with their own gene-seed. It would also imply the same for other Chaos Space Marines legions.

Thousand Sons

Magnus was unquestionably the most profoundly mutated of the Emperor's Primarchs, both physically and psychically, and the Legion imprinted with his gene-seed reflected that with a high percentage of Thousand Sons manifesting some level of psychic ability. Early in the Legion's history a small, but significant percentage were prone to physical mutation, but in the wake of falling thrall to Tzeentch that percentage escalated wildly. The Rubric ended that forever for the battle brothers of the Thousand Sons, but the sorcerers who command those armoured shells still carry the gene-seed of their Daemon Prince, and wear their grotesque mutations proudly as tokens of their mercurial patron's favour.

From the Fandom article on the Thousand Sons

There are still TS marines with the gene-seed. But it would seem that they're mutated and with the article on Fabrikus, it can be assumed that they can't create any more Space Marines from their original stock.

Answer 2

All the legions can create new marines to one extent or another, with the possible exception of the Thousand Sons. The Thousands Sons might too, as there's a short story involving an old Iron Warrior talking to other traitor marines (A Night Lord, a Thousand Sons sorcerer, and an Alpha Legionnaire) and discovers ALL of the other three marines were created post-heresy. (I believe the story is in the War Without End anthology, though I forget the actual name.) But that story, despite being fairly recent, might clash with the "dusty" nature of the current Tsons fluff. It was heavily implied though that all came from their "parent" legions, and were neither castoffs nor made with stolen geneseed.

The thing to remember with Chaos marines is that the Warp is, well, Chaotic. The Night Lords Trilogy, for example, has only 150 years passing for the Night Lords involved, despite it being in the 41st Millenium. So they only have 150 years of mutations to worry about, and having spent a good amount of that time "out and about" away from the Eye a lot of their geneseed should be usable. The same can be said for other warbands from other Legions. Sure the "average" Emperor's Children or World Eater marine is going to be 1,000s of years gone with geneseed that's worthless, but there's enough science/magic/weird luck described in the Chaos Legions that a limited amount of workable geneseed is almost guaranteed. Though given their proclivities when it comes to stealing geneseed from others I doubt it's enough to keep up with combat losses.

That's not to say entire legions have the capability to make new marines. They don't. I would go so far as to say MOST don't have the capability to make their own geneseed work. Warbands of the same Legion have wildly different capabilities both in mutations and geneseed recovery.