I feel that I cannot completely answer the question but that there is enough here to at least somewhat sort out the situation:
- WSL2 was first released as part of Windows 10 version "2004"
- Windows Server 2019 (current LTSC version) is version "1809"
- It was recently confirmed that WSL2 will be backported to Windows 10 versions "1903" and "1909"
- The Windows Server Semi-Annual Channel (SAC), which I believe only carries the Server Core and Nano Server installation types, essentially matches the Windows 10 releases and does have the "2004" Windows version
If what you are looking for is support in Windows Server 2019 (Windows version "1809") there has to my knowledge been no word that WSL2 will be backported to this Windows version (for either Windows 10 or Windows Server).
For the LTSC channel I would expect (based on lack of other information) that it will not be backported to Server 2019, but that the next Windows Server LTSC version (which will be based on something newer than "2004") ought to have it. The popular assumption seems to be the next version will be named "Server 2022", but I'm not sure this is actually confirmed.
WSL2 in current Server versions in SAC, like Server Core "2004" or Nano Server "2004", may well already be a thing, assuming that WSL is installable in the Core/Nano Windows installations(?).