Mark Rosewater (@Maro254) introduced the team members that tested War of the Spark and also gives a breakdown of how the Proliferate mechanic started up way back in Scars of Mirrodin. This is a great read covering other places that Proliferate was considered, its applications and they allude to Infect as well. You can read the full article below.
Basically, proliferate missed out on Kaladesh & Aether Revolt as well as missing out on being the Simic Mechanic due to comparative power level. With War of the Spark however it made perfect sense with the great number of Planeswalkers. The article indicates that Scars had 14 Proliferate cards in the entire block. And that War of the Spark has 14 in just this one set! So, proliferate is around till October 2020. And keep your eyes peeled for all 14 proliferate cards to get spoiled.
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