@mwhite 

Simple answer: ESXi 6.7 U2 

Elaborate one below

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You can look at the following link

https://portal.nutanix.com/#/page/compatibilitymatrix

This compatibility matrix shows the AOS, AHV, and hypervisor compatibility for Nutanix NX and SX Series platforms and SW-Only Models qualified by Nutanix (such as Cisco UCS, Dell PowerEdge, HPE ProLiant and others listed here). 

However for XC platfrom we have following note 

For other platforms not listed here, such as Dell XC, Lenovo HX, and others, please see your vendor documentation for compatibility.

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However, liked to go an extra mile to share following details to you. The latest according to the following link is ESXi 6.7 U2

Dell EMC XC630 Series Hyper-Converged Appliance Support Matrix (Updated Feb 2020)

https://downloads.dell.com/manuals/all-products/esuprt_converged_infrastructure/esuprt_hyperconverged_systems/dell-xc630_support-matrix_en-us.pdf

HTH