Openwrt 18.06 why not support the Xilinx ZYNQ chip?
The zynq target was not included on 18.06 because at the time 18.06 was branched, the target wasn't in good shape. It was using an old (4.4) kernel and the images weren't booting see FS#1204.
I reworked the image creation steps with commit dfac3ff and fixed the issue, but that commit didn't make it for 18.06.
Since then, I've been improving the target, updating the kernel and adding sdcard image support among other things.
I plan to keep improving the target. I'm now testing some patches to enable devicetree overlay support and be able to program the FPGA fabric.
You can download snapshots releases from https://downloads.openwrt.org/snapshots/targets/zynq/generic/
Is there any reason you need zynq support for 18.06?
More testers are always appreciated 
will it release next version?
There looks like there's source code there at https://github.com/openwrt/openwrt/tree/openwrt-19.07/target/linux/zynq
It's questionable, given the specific nature of this device class, why anyone would really WANT a "release" that they can just download off of the site.
It's almost guaranteed they would be doing something with the FPGA fabric (otherwise why bother with Zynq?), which would mean they'd best be served building from source.
Yes, it's on the 19.07 branch, and snapshot images are being generated.
Theoretically you could program the FPGA fabric by installing a package, but such packages don't exist on OpenWRT (yet).
One possible implementation for that is using Device Tree Overlays + some scripts.
That capability is currently not available, as Device Tree Overlays is not on mainline kernel nor on the zynq target. I did an early testing of some patches that I took from the raspberry pi kernel and it works, but it needs more testing and validation.