hashes: Deprecate sha256t_tag macro

diff --git a/hashes/src/macros.rs b/hashes/src/macros.rs
index b8dd7e28..9f3dbd83 100644
--- a/hashes/src/macros.rs
+++ b/hashes/src/macros.rs
@@ -10,25 +10,34 @@
 
 /// Macro used to define a tag.
 ///
-/// Defines new struct and implements [`Tag`](crate::sha256t::Tag) for it.
+/// # Deprecated
 ///
-/// The syntax is:
+/// This macro is deprecated in favor of implementing [`Tag`](crate::sha256t::Tag) by hand.
+/// [`Midstate::hash_tag`](crate::sha256::Midstate::hash_tag) and
+/// [`Midstate::new`](crate::sha256::Midstate::new) are `const fn`, so a tag's midstate can be
+/// computed in a `const` context without any macro:
 ///
 /// ```
-/// # use bitcoin_hashes::sha256t_tag;
-/// sha256t_tag! {
-///     /// Optional documentation details here.
-///     /// Summary is always generated.
-///     pub struct FooTag = hash_str("foo");
+/// use bitcoin_hashes::{sha256, sha256t};
+///
+/// /// The tag for `FooHash`.
+/// #[derive(Copy, Clone, PartialEq, Eq, Default, PartialOrd, Ord, Hash)]
+/// pub struct FooTag;
+///
+/// impl sha256t::Tag for FooTag {
+///     const MIDSTATE: sha256::Midstate = sha256::Midstate::hash_tag(b"foo");
 /// }
 /// ```
+/// 
+/// The macro's three constructors map to these `MIDSTATE` expressions:
 ///
-/// The `hash_str` marker says the midstate should be generated by hashing the supplied string in a
-/// way described in BIP-0341. Alternatively, you can supply `hash_bytes` to hash raw bytes. If you
-/// have the midstate already pre-computed and prefer **compiler** performance to readability you
-/// may use `raw(MIDSTATE_BYTES, HASHED_BYTES_LENGTH)` instead, note that `HASHED_BYTES_LENGTH` must
-/// be a multiple of 64.
+/// - `hash_str("foo")` becomes `sha256::Midstate::hash_tag("foo".as_bytes())`
+/// - `hash_bytes(b"foo")` becomes `sha256::Midstate::hash_tag(b"foo")`
+/// - `raw(BYTES, LEN)` becomes `sha256::Midstate::new(BYTES, LEN)`
+///
+/// This macro dates to a time before SHA256 could be computed in a `const` context.
 #[macro_export]
+#[deprecated(since = "TBD", note = "use `sha256t::Tag` instead")]
 macro_rules! sha256t_tag {
     ($(#[$($tag_attr:tt)*])* $tag_vis:vis struct $tag:ident = $constructor:tt($($tag_value:tt)+);) => {
         $crate::sha256t_tag_struct!($tag_vis, $tag, stringify!($tag), $(#[$($tag_attr)*])*);
@@ -567,6 +576,8 @@ macro_rules! impl_serde_traits(
 
 #[cfg(test)]
 mod test {
+    #![allow(deprecated_in_future)]
+
     use crate::sha256;
 
     #[test]
diff --git a/hashes/src/sha256t/mod.rs b/hashes/src/sha256t/mod.rs
index 45848e06..e69a9792 100644
--- a/hashes/src/sha256t/mod.rs
+++ b/hashes/src/sha256t/mod.rs
@@ -192,6 +192,8 @@ macro_rules! sha256t_tag_constructor {
 
 #[cfg(test)]
 mod tests {
+    #![allow(deprecated_in_future)]
+
     #[cfg(feature = "alloc")]
     #[cfg(feature = "hex")]
     use crate::sha256;
diff --git a/hashes/tests/api.rs b/hashes/tests/api.rs
index 76c1fa21..3784e66c 100644
--- a/hashes/tests/api.rs
+++ b/hashes/tests/api.rs
@@ -8,6 +8,7 @@
 
 #![allow(dead_code)]
 #![allow(unused_imports)]
+#![allow(deprecated_in_future)]
 // Exclude lints we don't think are valuable.
 #![allow(clippy::uninlined_format_args)] // Allow `format!("{}", x)` instead of enforcing `format!("{x}")`