Evidence? Trojet,show me one single ancient Slavic I2a1b............!
To date the majority of ancient DNA samples come from before I-CTS10228 aka I2a-Slavic mutation TMRCA of 2300 ybp even existed, hence the chances of finding one are pretty slim.
However, go get your Slavic archeologists and test some remains in the area north of the Carpathians from 0-500 AD and you likely will find plenty of I-CTS10228+ samples there.
We recently had the Genomic History of Southeastern Europe, and no I2a1-M423 (where I2a-Slavic stems from) was found anywhere in the Balkans which sampled from the Mesolithic all the way to the Iron Age, but to the contrary I2a1-M423 was found all over north of the Balkans. Instead what we had in the heart of Bronze Age Dalmatia (Croatia), is J2b2a-L283 as can be seen here.