Rename misused "deprecation" to removal (#10000)

# Description
In the past we named the process of completely removing a command and
providing a basic error message pointing to the new alternative
"deprecation".

But this doesn't match the expectation of most users that have seen
deprecation _warnings_ that alert to either impending removal or
discouraged use after a stability promise.

# User-Facing Changes
Command category changed from `deprecated` to `removed`

crates/nu-command/src/removed/format.rs

@@ -0,0 +1,46 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, PipelineData, ShellError, Signature, SyntaxShape, Type};
+
+#[derive(Clone)]
+pub struct SubCommand;
+
+impl Command for SubCommand {
+    fn name(&self) -> &str {
+        "date format"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("date format")
+            .input_output_types(vec![
+                (Type::Date, Type::String),
+                (Type::String, Type::String),
+            ])
+            .allow_variants_without_examples(true) // https://github.com/nushell/nushell/issues/7032
+            .switch("list", "lists strftime cheatsheet", Some('l'))
+            .optional(
+                "format string",
+                SyntaxShape::String,
+                "the desired date format",
+            )
+            .category(Category::Removed)
+    }
+
+    fn usage(&self) -> &str {
+        "Removed command: use `format date` instead"
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Err(nu_protocol::ShellError::RemovedCommand(
+            self.name().to_string(),
+            "format date".to_owned(),
+            call.head,
+        ))
+    }
+}

crates/nu-command/src/removed/let_env.rs

@@ -0,0 +1,43 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{Category, PipelineData, ShellError, Signature, SyntaxShape, Type};
+
+#[derive(Clone)]
+pub struct LetEnv;
+
+impl Command for LetEnv {
+    fn name(&self) -> &str {
+        "let-env"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name())
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .allow_variants_without_examples(true)
+            .optional("var_name", SyntaxShape::String, "variable name")
+            .optional(
+                "initial_value",
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
+                "equals sign followed by value",
+            )
+            .category(Category::Removed)
+    }
+
+    fn usage(&self) -> &str {
+        "`let-env FOO = ...` has been removed, use `$env.FOO = ...` instead."
+    }
+
+    fn run(
+        &self,
+        _: &EngineState,
+        _: &mut Stack,
+        call: &Call,
+        _: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Err(nu_protocol::ShellError::RemovedCommand(
+            self.name().to_string(),
+            "$env.<environment variable> = ...".to_owned(),
+            call.head,
+        ))
+    }
+}

crates/nu-command/src/removed/mod.rs

@@ -0,0 +1,7 @@
+mod format;
+mod let_env;
+mod removed_commands;
+
+pub use format::SubCommand as DateFormat;
+pub use let_env::LetEnv;
+pub use removed_commands::*;

crates/nu-command/src/removed/removed_commands.rs

@@ -0,0 +1,15 @@
+use std::collections::HashMap;
+
+/// Return map of <removed_command_name, new_command_name>
+/// This covers simple removed commands nicely, but it's not great for deprecating
+/// subcommands like `foo bar` where `foo` is still a valid command.
+/// For those, it's currently easiest to have a "stub" command that just returns an error.
+pub fn removed_commands() -> HashMap<String, String> {
+    [
+        ("fetch".to_string(), "http get".to_string()),
+        ("post".to_string(), "http post".to_string()),
+        ("benchmark".to_string(), "timeit".to_string()),
+    ]
+    .into_iter()
+    .collect()
+}