diff --git a/ruma-identifiers-macros/Cargo.toml b/ruma-identifiers-macros/Cargo.toml
index 3c897f4b..8f8c7be8 100644
--- a/ruma-identifiers-macros/Cargo.toml
+++ b/ruma-identifiers-macros/Cargo.toml
@@ -10,7 +10,7 @@ version = "0.18.1"
 edition = "2018"
 
 [dependencies]
-ruma-identifiers-validation = { version = "0.2.2", path = "../ruma-identifiers-validation", default-features = false }
+ruma-identifiers-validation = { version = "0.2.3", path = "../ruma-identifiers-validation", default-features = false }
 quote = "1.0.8"
 proc-macro2 = "1.0.24"
 syn = "1.0.55"
diff --git a/ruma-identifiers-validation/CHANGELOG.md b/ruma-identifiers-validation/CHANGELOG.md
index 9b81cebf..37997d6b 100644
--- a/ruma-identifiers-validation/CHANGELOG.md
+++ b/ruma-identifiers-validation/CHANGELOG.md
@@ -1,3 +1,11 @@
+# 0.2.3
+
+Improvements:
+
+* Add a `compat` feature
+  * Under this feature, more user IDs are accepted that exist in the while but are not
+    spec-compliant
+
 # 0.2.2
 
 Improvements:
diff --git a/ruma-identifiers-validation/Cargo.toml b/ruma-identifiers-validation/Cargo.toml
index 2c94fb8e..b9b267f9 100644
--- a/ruma-identifiers-validation/Cargo.toml
+++ b/ruma-identifiers-validation/Cargo.toml
@@ -8,5 +8,8 @@ authors = [
     "Jonas Platte <jplatte@posteo.de>",
 ]
 license = "MIT"
-version = "0.2.2"
+version = "0.2.3"
 edition = "2018"
+
+[features]
+compat = []
diff --git a/ruma-identifiers-validation/src/user_id.rs b/ruma-identifiers-validation/src/user_id.rs
index c76938e2..ae20f814 100644
--- a/ruma-identifiers-validation/src/user_id.rs
+++ b/ruma-identifiers-validation/src/user_id.rs
@@ -14,6 +14,11 @@ pub fn validate(s: &str) -> Result<(NonZeroU8, bool), Error> {
 ///
 /// Returns an `Err` for invalid user ID localparts, `Ok(false)` for historical user ID localparts
 /// and `Ok(true)` for fully conforming user ID localparts.
+///
+/// With the `compat` feature enabled, this will also return `Ok(false)` for invalid user ID
+/// localparts. User IDs that don't even meet the historical user ID restrictions exist in the wild
+/// due to Synapse allowing them over federation. This will likely be fixed in an upcoming room
+/// version; see <https://github.com/matrix-org/matrix-doc/pull/2828>.
 pub fn localpart_is_fully_comforming(localpart: &str) -> Result<bool, Error> {
     // See https://matrix.org/docs/spec/appendices#user-identifiers
     let is_fully_conforming = localpart
@@ -23,9 +28,13 @@ pub fn localpart_is_fully_comforming(localpart: &str) -> Result<bool, Error> {
     // If it's not fully conforming, check if it contains characters that are also disallowed
     // for historical user IDs. If there are, return an error.
     // See https://matrix.org/docs/spec/appendices#historical-user-ids
+    #[cfg(not(feature = "compat"))]
     if !is_fully_conforming && localpart.bytes().any(|b| b < 0x21 || b == b':' || b > 0x7E) {
         Err(Error::InvalidCharacters)
     } else {
         Ok(is_fully_conforming)
     }
+
+    #[cfg(feature = "compat")]
+    Ok(is_fully_conforming)
 }
diff --git a/ruma-identifiers/CHANGELOG.md b/ruma-identifiers/CHANGELOG.md
index 1b2381bc..eb837c86 100644
--- a/ruma-identifiers/CHANGELOG.md
+++ b/ruma-identifiers/CHANGELOG.md
@@ -7,6 +7,12 @@ Breaking changes:
   * Use `MxcUri::is_valid` to make sure it is spec-compliant
   * `MxcUri::{media_id, server_name}` return `Some({value})` only if the URI is spec-compliant
 
+Improvements
+
+* Add a `compat` feature
+  * Under this feature, more user IDs are accepted that exist in the while but are not
+    spec-compliant
+
 # 0.18.1
 
 Improvements:
diff --git a/ruma-identifiers/Cargo.toml b/ruma-identifiers/Cargo.toml
index 810bc9ab..744cbe57 100644
--- a/ruma-identifiers/Cargo.toml
+++ b/ruma-identifiers/Cargo.toml
@@ -20,6 +20,7 @@ rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
 default = ["serde"]
+compat = ["ruma-identifiers-validation/compat"]
 serde = ["ruma-serde", "serde1"]
 
 [dependencies]
@@ -27,7 +28,7 @@ either = { version = "1.6.1", optional = true }
 paste = "1.0.4"
 rand = { version = "0.8.0", optional = true }
 ruma-identifiers-macros = { version = "=0.18.1", path = "../ruma-identifiers-macros" }
-ruma-identifiers-validation = { version = "0.2.2", path = "../ruma-identifiers-validation", default-features = false }
+ruma-identifiers-validation = { version = "0.2.3", path = "../ruma-identifiers-validation", default-features = false }
 ruma-serde = { version = "0.3.1", path = "../ruma-serde", optional = true }
 ruma-serde-macros = { version = "0.3.1", path = "../ruma-serde-macros" }
 # Renamed so we can have a serde feature.
diff --git a/ruma-identifiers/src/user_id.rs b/ruma-identifiers/src/user_id.rs
index 10e885ae..f172707c 100644
--- a/ruma-identifiers/src/user_id.rs
+++ b/ruma-identifiers/src/user_id.rs
@@ -239,6 +239,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(not(feature = "compat"))]
     fn invalid_characters_in_user_id_localpart() {
         assert_eq!(UserId::try_from("@te\nst:example.com").unwrap_err(), Error::InvalidCharacters);
     }
diff --git a/ruma/Cargo.toml b/ruma/Cargo.toml
index 654ce297..f1f4d2ad 100644
--- a/ruma/Cargo.toml
+++ b/ruma/Cargo.toml
@@ -86,7 +86,12 @@ full = [
 #
 # For example, some mandatory string fields are defaulted to an empty string if
 # missing with this feature.
-compat = ["ruma-common/compat", "ruma-events/compat", "ruma-client-api/compat"]
+compat = [
+    "ruma-common/compat",
+    "ruma-events/compat",
+    "ruma-identifiers/compat",
+    "ruma-client-api/compat",
+]
 
 # unstable: by using any of these, you opt out of all semver guarantees Ruma
 #           otherwise provides!