I don't see a 18.0 win86 version so i think you might be stuck with this warning. (or you need to build v18.0 yourself, but i'm not sure if that's possible either.)
According to achow101, there's 32-bit/x86 librar for Bitcoin Core. You just need to build/make/compile it by yourself.
The Windows 32-bit has had some performance and other issues. While it still works, it does also hold back some other future improvements. By removing it now, we won't have to worry about supporting Windows 32-bit for those future improvements. You can read the discussion about this here:
http://www.erisian.com.au/meetbot/bitcoin-core-dev/2019/bitcoin-core-dev.2019-04-11-19.00.log.html (it's during the Platform Deprecation topic towards the end).
It was removed under the assumption that few or no people were using Windows 32-bit.
Note that Bitcoin Core 0.18.0 still had Windows 32-bit binaries built. They simply weren't uploaded for now. If there is enough people who are using Windows 32-bit, they can and will be uploaded too and future releases will have them as well.
