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How far is the geneseed of these 2 legions gone at present?

I know the 1k sons had serious defects prior to the heresy even, and I doubt Nurgles rot did any good to the Death guard geneseed.

Basically, can either of these legions create new marines from their own geneseed?

Thousand Sons are all rubric marines: just a bit of dust in armor.

I don't know any further about it, but I assume the Thousand sonds don't really do geneseed anymore, but instead turn traitor astarte in rubric marines with a ritual.

The geneseed of most traitor legions and CSM degenerates in the warp. This is why they raid loyalists for geneseed that isnt tainted by the warp (Iron Warriors, Red Corsairs). So imo these two legions you mentioned are some of the worst affected.
The Thousand Sons are easy to awnser, they have almost no marines to get geneseed from. They have all been turned to dust, except for the sorcerers. They lack the geneseed or the apothecaries to make new marines from their own legion.
Death Guard may not even have geneseed in their bodies anymore, or in usable form. The effect of the rot is a bit different from marine to marine. But implanting such a disease via implant would probably quickly kill a normal human without marine implants and the blessing of Nurgle.