Customers of Optics Planet (https://www.opticsplanet.com) found out recently they could no longer buy magazines with standard capacity. The website only offfered 10 round magazines for their customer as of 2/19/2018. One user reportedly recieved this email from Optics Planet:
People will react differently in tragedies like the recent shooting in FL and there will be knee-jerk reactions, but it's surprising that a popular firearm accessory online retailor would make such a drastic move that may upset their customer base. There hasn't been an official press release from Optics Planet and we are awaiting for their reply on this matter so we do not know what the official report is. We will update the story as it unfolds.
***UPDATE*** Optics Planet explained that their warehouse is located in IL which prohibits magazines bigger than 10 rounds.