I've had more than a handful of clients do this without issue.
Same here... I don't think that there's been a formal announcement for Windows Server 2019 compatibility, but certainly we've got clients using it on a RDS.
Formal announcement on SQL Server 2019 in v5.4 build 1.09 back in November 2018 here: https://www.quotewerks.com/updates/UpdateAgent.asp?VersionBuild=5.4.01&Version=5.4&Build=1
We want to upgrade our RDS box to Win Server 2019. Before we do, can someone confirm that QW will work nicely with this?