Since this isn't the first time this has happened, it would be nice to know what the official policy is with this problem.
XCode 8 El Capitan Mac OSX 10.11.6 Part #1. The latest Xcode running under 10.11.6 would be Xcode 8.2.1. You can download Xcode 8.2.1 on apple developers site. I do not remember which SDKs Xcode 8.2.1 comes.
Running latest 10.11, installed xcode-8, as software update suggested I do.
kext no longer compile/load due to the message:
Sep 14 14:06:35 icgi-vip kernel[0]: kxld[net.lundman.zfs]: The super class vtable '__ZTV20IOBlockStorageDevice' for vtable '__ZTV27net_lundman_zfs_zvol_device' is out of date. Make sure your kext has been built against the correct headers.
Xcode 10.1 Xip
Right, since it compiles against /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk/System/Library/Frameworks/Kernel.framework/Headers
Xcode 10.13
I guess that is true. But of course xcode deletes the MacOSX10.11.sdk entry, so there is no way to compile kext for the OS I am on? This seems a strange situation to put developers into.
Xcode 10.15.2
I can copy MacOSX10.11.sdk from another host, but presumably Apple has something else in mind here.