Socketed. LGA 1151 Gen 3 for Coffee Lake support which is the same dimensionally and pin out as the previous LGA 1151 on Kaby Lake iMacs just with additional pin enabled for power and such.
Its not a complete rip off from apple, the i5-8600 is $236 real world retail, Apple ask $500 for the 9900k which is $526 real world. So $200 dollars Apple Tax (i5 $236 - i9 $526 = $290, upgrade $500 - $290 = $210). In that respect the high end iMac i5 to i9 is a better deal since the upgrade is $400 from the i5 which is similar in price.
If you haven't before its a pain to upgrade. Pending design changes you'll need to remove the logic board. I would consider that the easy part though, reinstalling the heat sink is the part I always dread. The heat sync covers the GPU and CPU, the CPU is held in place by the heat sink and the GPU spring tension can be defined as "intense". Just make sure the CPU is and heat sink is secure then work on the GPU, its easy for the CPU to move and bend the pins of the LGA if you are working on the GPU first. Due to the logic board design its impossible to clamp it without speciality tools.