@Riverman
The 0 growth period you mentioned corresponds with the massacres and resettlements after the Great Illyrian Revolt.
But with the data at hand, it should be J2b2 the decimated one and resettled in Eastern Balkans or taken as slaves elsewhere.
It could be different tribes, but it only corresponds to the first downturn - after which there was a very small uptick even. This is 200-300 years earlier and affects both E-V13 and J-L283, which could correspond either to the Illyrian or Thracian uprisings:
During the Macedonian Wars, conflict between Rome and Thrace was unavoidable. The rulers of Macedonia were weak, and Thracian tribal authority resurged. But after the Battle of Pydna in 168 BC, Roman authority over Macedonia seemed inevitable, and the governance of Thrace passed to Rome.[citation needed] Initially, Thracians and Macedonians revolted against Roman rule. For example, the revolt of Andriscus, in 149 BC, drew the bulk of its support from Thrace. Incursions by local tribes into Macedonia continued for many years, though a few tribes, such as the Deneletae and the Bessi, willingly allied with Rome.
https://en.wikipedia.org/wiki/Thracians#Roman_rule
The really big blow corresponds in time not with the J-L283 growth and not with the Illyrian stories, but only with the Dacian wars, Sarmatian and Germanic invasions. And I'd really say the Dacians got hit much harder than the Illyrians in the Roman era. Germanic and Slavic, maybe different, because they seem to have allied up with them in the forms of some Dacian and Daco-Sarmatian groups.
If you look at the other patterns, a time gap of 200-300 years of one founding population is something which doesn't exist in the data. I was astonished by this fact myself, because my trust into YFull estimates was not that high before. But there is something about it.