Can I have a sql 2017 read-scale AG on different Windows server OSs; i.e. Windows server 2016/2019?
Availability Groups are built upon Windows Failover Cluster.  As such, it won't (probably?) be a supported scenario to run long term with different OS versions in the same cluster.  Assuming it lets you set it up in the first place.
You might ask in System Administration if it's possible to setup a windows cluster between a OS 2016 and 2019.  If you can and if it's supported, then SQL won't have any issues.
You could do a distributed Availability Group as that is not built upon Windows Failover Cluster, but you lose a lot of advantages (no listener, to start with).