According to an IEM Oaklnd source, the event will be played on Patch 6.21
Patch 6.21 is the last version of League of Legends in season 6. This means that the Patch 6.22 which is the first patch in the 2017 Pre-season will be put aside in this major tournaments. Patch 6.22 has yet to be hosted in any major tournament, likely due to the unbalances from the changes.
This is a welcomed change as players have yet to adjust to Patch 6.22, and Patch 6.21 is much more suitable for competitive play since it is much more balanced and refined. Sure seeing the plants in competitive play would be extremely entertaining as the coordination of out of vision plays and cision control would be much stronger, however, many feel that the addition is a nuisance.
The event will also only feature 6 teams compared to the original 8, partly due to the decision by teams to reject the invitation to the event. Team SoloMid have announced they will be attending the event, and will also feature their new ADC at the event.