Why was the 7th model Conscript upgraded introduced a few patches ago?
Because the lack of non-doctrinal weapon upgrades for Conscripts resulted in abhorrent late game scaling, as well as the inability to purchase and upgrade a vet 0 Conscript squad in the late game because it would get absolutely nuked by veteran Axis infantry (especially elite infantry) with weapons upgrades, resulting in very unfair and one-sided situations were late game wipes on veteran conscripts were game deciding. Thus the balance team had 2 options, either to give them a weapon upgrade such as a double/single DP28 LMG, or to increase the squad size to 7 and provide rate of fire bonuses. They went with the later, as well as reload and veterancy gain bonuses to allow the Soviet player to purchase Conscripts in the late game and not be unfairly punished for it.
What event made that "improvement" come true?
It wasn't really a single event, more of an issue with players constantly going T1 for Penals and choosing not to bother with conscripts. The Soviet faction did not have usable mainline infantry, and to the balance team's credit they identified and fixed the issue.
Who requested it?
About 90% of the playerbase and about 99% of the elite 1v1 players. The general consensus before the May 2019 Balance patch was that Conscripts were in dire need of late game scaling and some kind of upgrade was necessary. Feel free to read up on that:
https://www.coh2.org/topic/89485/soviet-general-faction-changes-new-cmdr-mod-5-0
Anything else?
Greece
2
Stormjäger
Comrade Dyatlov
DaPopeCOH
Stormjäger
Relic Entertainment
보드카 중대 
