Doesn't completely answer the question since we still need Ivanti to update the supported platforms reference document but they did post this: New Ivanti Endpoint Manager Content Available - 10/04/2018
In there, Windows Server 2019 content is included.
In general you will find that the newer OS will work as an endpoint but there 'might' be some issues as they haven't fully tested against it. If Ivanti can answer here that's great but also maybe ask the question directly of support to get their response since being supported is the critical part.
Don't expect 2019 as a Core anytime soon though
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To fill in a few blanks, as I've had to clarify this myself.
As a Core Server -- not presently supported (since the OS came out after release). Likely to be supported with EPM 2019.x
As a client OS -- full support should be added with 2018.3 SU2 (which should go into limited release in a few weeks), unless there's more problems with it (either the SU or Windows Server 2019).
Official support will be reflected in - Supported Platforms and Compatibility Matrix for LANDESK Management Suite/Ivanti Endpoint Manager - once it's added in. But the above should help lay down some expectations at least.
(For what it's worth, if you need to do some early acceptance testing, a 2018.3 + SU1 client *will* install, but will report it back as a Server 2016 OS).