Apologies for the delayed reply. Have been working on and submitted a grant application for the 2.1 plugin upgrade: Upgrade BTCPayServer Zcash Plugin for 2.1
Yes, initially Sapling shielded addresses will be supported. Orchard and UA should be supported pending an upgrade of zcash-walletd, which can either be worked on in parallel or after the release of the plugin for 2.1, @hanh had offered to handle this as part of a grant application. Should be a seamless upgrade once it is ready.