The basic assumption, which a couple of archaeologists hold up too, is that the Proto-Thracian culture was Channelled Ware/G�va and if you might notice, I wrote about it here:
https://www.eupedia.com/forum/threa...an-case/page12?p=637970&viewfull=1#post637970
Proto-Slavs had early contacts with Channelled Ware, both directly and indirectly, via Lusatians. You can read up on contacts with Lusatians and Trzciniec. The article is linked in the post, but this is the direct link:
https://www.academia.edu/32255479/T...ron_Age_Settlement_at_Wierzchosławice_Site_15
The very same elements reached Greece and Asia minor (Troy), later Thracian territories with Belegis II-G�va along the Morava valley, and from the East the Fluted Ware horizon and Knobbed Ware in the Lower Danube, from where they entered Asia minor. In all those places Daco-Thracian place names being testified:
https://en.wikipedia.org/wiki/List_of_ancient_Daco-Thracian_peoples_and_tribes
As you can see, much of Northern Greece was "Grecisized" but was originally Thracian. Channelled Ware elements moved deep into Illyrian and Greek territory.
E.g., one of the signatures for Channelled Ware people being flame shaped, fully casted spearheads and Reutlingen/Naue II type swords in areas which were devoid of it earlier, as well as black burnished, channelled/fluted and knobbed fine ceramic, together with cremation in urns. You find it all from Transcarpathia/Southern Poland down to Asia minor in the LBA-EIA transition and in all places in which a related culture persisted, you find Daco-Thracians. In areas which had elements of it, at least substrate effects, like in the Dardanians.
Its a pretty clear cut case overall, even more so for the succeeding main cultures of Bosut-Basarabi and Psenichevo. While for Channelled Ware there might remain a small doubt, for the horizon as a whole, Psenichevo-Basarabi is absolutely clearly and 100 % both heavy in E-V13 and Daco-Thracian ethnolinguistically. Whatever was before, which is still somewhat in the dark, that's pretty certain and no longer a matter of debate.
Yes, they were already present in other people too, even in the LBA the latest, but that was largely due to contacts with Channelled Ware and the great influence Channelled Ware/Psenichevo-Basarabi had on other groups, including intensive, proven contacts, like with Illyrians, Greeks and Celts, presumably also with Veneti, Ligurians and Pre-Slavs among others. The pre-Slavic groups might have been their neighbours to the North/North East, in Eastern Poland and Ukraine.
Daco-Thracian is supposed to be linguistically closest to Balto-Slavs and Channelled Ware emerged from Epi-Corded groups living in the Carpathian zone.