Free Servers are servers that are accessible to all users regardless of their rank.
Free Servers were technically introduced the day Mineplex came out. However, in a way they got introduced on Friday, August 1st to conform to the EULA when Premium Servers got removed. Full changes made to conform to the EULA can be viewed at EULA Changes. Free Servers only allow access to the Free and Gem locked kits due to the EULA restriction regarding kits and how they may be made available. Therefore even ranked players must purchase the Gem locked kits despite their rank.