"You are ineligible to earn loot due to previous behavioral restrictions" - official response
I received a 10-game chat restriction last week, and played through my 10 games without incident. After the chat restriction was lifted I found that, although I was regularly receiving S grades, I was not eligible for loot. This is not clearly explained by Riot anywhere that I could find so I opened a ticket. Here is the response:
It looks like you've recently been punished for behavioral issues. Players who've been punished will no longer earn Hextech Chests through achieving an S rank on the Rift, nor will they earn key fragments after matchmade victories. This restriction will be lifted after your punishment ends and you've shown clear improvement. Because it's based on your personal behavior, there's no set date or time when this will happen.
Frost Ace Player Support Specialist The Tidecaller's Voice
There is no way to know when, or more likely if, a punished player is eligible for Hextech loot again. Seems alarmingly harsh for offenses that aren't outright bans.