Yesterday, Microsoft released Windows 10 Insider Preview build 18362.30 to the Release Preview ring, marking it as the release candidate for the May 2019 Update. Today is Patch Tuesday, and while all supported versions of the OS got a patch today, so is the latest Insider Preview.
The update is KB4495666, and it brings the build number to 18362.53. While this is a preview, Microsoft actually released a knowledge base article, which is rare for previews. Moreover, you can manually download the update from the update catalog.
Unfortunately, there's no specific changelog showing what's new. It only says that it contains "security updates that come as part of the normal monthly Patch Tuesday release cycle." Presumably, you can get a better idea of what this includes by checking out the release notes from the production builds that went out today.
There's also one known issue to be aware of:
|After installing this update, users may experience error “0x800705b4” when launching Windows Defender Application Guard or Windows Sandbox.||
Use the credentials of a local admin to create and set the following registry keys on the Host OS then restart the Host:
Microsoft is working on a resolution and will provide an update in an upcoming release.
The update should only be for those that are on any ring (Fast, Slow, or Release Preview) of the Windows Insider Program. If you unenrolled your PC, you shouldn't get it.