Add deprecation notices for old terms

See: #748

user/reference/glossary.md

@@ -91,7 +91,10 @@ Synonymous with [TemplateBasedVM](#templatebasedvm).
 
 NetVM
 -----
-Network Virtual Machine. 
+*This is an old definition from before Qubes 4.0.
+NetVMs, as defined here, no longer exist in Qubes 4.0 or later (see [here][pr-748] for technical details).*
+
+Network Virtual Machine.
 A type of [VM](#vm) that connects directly to a network.
 Other VMs gain access to a network by connecting to a NetVM (usually indirectly, via a [FirewallVM](#firewallvm)).
 A NetVM called `sys-net` is created by default in most Qubes installations.
@@ -101,12 +104,18 @@ For example, if `untrusted` is directly connected to `sys-firewall` for network
 
 ProxyVM
 -------
+*This is an old definition from before Qubes 4.0.
+ProxyVMs, as defined here, no longer exist in Qubes 4.0 or later (see [here][pr-748] for technical details).*
+
 Proxy Virtual Machine. 
 A type of [VM](#vm) that proxies network access for other VMs. 
 Typically, a ProxyVM sits between a NetVM and another VM (such as an AppVM or a TemplateVM) that requires network access.
 
 FirewallVM
 ----------
+*This is an old definition from before Qubes 4.0.
+FirewallVMs, as defined here, no longer exist in Qubes 4.0 or later (see [here][pr-748] for technical details).*
+
 Firewall Virtual Machine. 
 A type of [ProxyVM](#proxyvm) that is used to enforce network-level policies (a.k.a. "firewall rules"). 
 A FirewallVM called `sys-firewall` is created by default in most Qubes installations.
@@ -186,3 +195,7 @@ QWT
 ----
 An abbreviation of Qubes [Windows Tools](#windows-tools).
 
+
+[pr-748]: https://github.com/QubesOS/qubes-doc/pull/748
+
+