The last build in the beta program from Essential was released earlier this month. It is based on Oreo 8.1 as the company is skipping 8.0 altogether. This change to OTA updating should make it much easier for those who want to join to do so. Sideloading, while still an option, is a tricky process and many simply don't want to do it.
There is no official word yet on when the beta program will end and the Essential Phone will get its official Oreo build, but it likely isn't more than a few months away depending on now the beta testing goes.