Iranian languages and Slavic languages have some similarities, such as the merger of aspirated sounds into unaspirated sounds, and the development of the consonant /z/. Historically, the settlements of the Scythians and Sarmatians were also close to proto-Slavs.
The migration time of the Indo-Aryans is still a mystery. Only the bottleneck Z2124 has been found in Sintashta, without L657 of the Indo-Aryans.
So I guess that the Iranian language was independent when it was separated from Proto-Indo-European and was closer to Balto-Slavic. The migration of the Indo-Aryans was quite early and rapid. In addition to entering South Asia, some insular tribes also entered the Iranian plateau, Afghanistan and Mitanni, and their language may be more conservative than Vedic.
Therefore, when the later Iranians migrated, they absorbed Indo-Aryans living outside South Asia. Avesta has a high probability of originating in Afghanistan, and we still have high-frequency L657 in Afghanistan. In this process, another fusion of language and culture is sure to take place. For example, Asko Parpola argues that the merger of e and o to a took place first in Indo-Aryan and then passed on to Iranian languages that entered Central Asia. It is also mentioned in his paper that the disappearance of Iranian aspirated stops may be related to the contact of North Caucasian/Balto-Slavic populations.
In contrast, the Scythian pantheon and culture are far removed from India and Iran. This is because they did not go south and experience massive linguistic and cultural integration with the first Indo-Aryans. Scythians do not call themselves Aryans, but another term related to "Skuda".