The term “Scythian” is as meaningless as “barbarian” — scythian being the generic term the Greeks gave to north-western barbarians (”barbarians” being the generic Greek term for non-Greeks!)
It’s as funny as the Zhou dynasty Chinese giving names to the groups of people around them (and we don’t know what these people called themselves as the Chinese/Huaxia people had writing and the others didn’t) - the “northern barbarians”, the “western barbarians”, the “southern barbarians” :)
Anyway, so the “Scyths” were just the generic terms for a whole buncha peoples - whether Iranian or Slavic or other
The Sarmatians are a later group of different Alani (Aryani) tribes who were definitively Irani speaking
Have they ever determined where the Slavs were 500 B.C.? Their language is in the "satem" group (sto for "100" in modern Slavic languages) but not part of the Indo-Iranian branch.
Beth-shean (west of the Jordan in Israel, within the pre-1967 "green line") was called Scythopolis for a while in antiquity. I don't know when that label started to be used.