The second/final milestone is now complete.
Version 1.0.2 of the BTCPay Zcash plugin has been published to the plugin repository, adding support for configuring the wallet viewing key, birth height and number of confirmations via the settings GUI. Source code: GitHub - btcpay-zcash/btcpayserver-zcash-plugin: This plugin extends BTCPay Server to enable users to receive payments via Zcash.
The official installation process is now supported using Docker fragments. There is an open PR in the upstream btcpayserver-zcash repository, and until it is merged, a fork can be used: GitHub - btcpay-zcash/btcpayserver-docker: Docker resources for hosting BTCPayServer easily
I have published some documentation on using the plugin with either a new BTCPayServer instance or existing instance (using a fork of btcpayserver-docker): btcpayserver-zcash-plugin/docs/installation.md at master · btcpay-zcash/btcpayserver-zcash-plugin · GitHub
I plan to continue maintaining the plugin and Docker repositories for at least a year provided that there are no serious breaking changes that require significant time and extra funding to fix. I will work on some improved documentation/guidance on contributing to the .NET plugin, for now there are basic steps for creating a local build and the official documentation for plugin development can be used: Local development | BTCPay Server and Plugins | BTCPay Server.