According to the compatibility list at ASUS, the i7-8700K requires BIOS version 1302.Basically, the i7-8700K is compatible with the ASUS XI code - whether with an older BIOS version depends on a try, or what sense is behind it.

A reduction on the set?? not sure I understand

see the manual on page: 1-15 and 1-16 as well as 1-17 https://dlcdnets.asus.com/pub/ASUS/mb/LGA1151/ROG_MAXIMUS_XI_CODE/G14941_ROG_MAXIMUS_XI_CODE_UM_V3_WEB.pdf

Unfortunately, the delivery time is still unknown.

Yes, it is compatible. See also the manufacturer's website: https://www.asus.com/us/Motherboards/ROG-MAXIMUS-XI-CODE/specifications/