use unstable prefix

src/crypto/index.js

@@ -1959,7 +1959,7 @@ async function _uploadOneTimeKeys(crypto) {
 
     const res = await crypto._baseApis.uploadKeysRequest({
         one_time_keys: oneTimeJson,
-        fallback_keys: fallbackJson,
+        "org.matrix.msc2732.fallback_keys": fallbackJson,
     });
 
     await crypto._olmDevice.markKeysAsPublished();

src/sync.js

@@ -1361,13 +1361,14 @@ SyncApi.prototype._processSyncResponse = async function(
         const currentCount = data.device_one_time_keys_count.signed_curve25519 || 0;
         this.opts.crypto.updateOneTimeKeyCount(currentCount);
     }
-    if (this.opts.crypto && data.device_unused_fallback_keys) {
+    if (this.opts.crypto && data["org.matrix.msc2732.device_unused_fallback_keys"]) {
         // The presence of device_unused_fallback_keys indicates that the
         // server supports fallback keys. If there's no unused
         // signed_curve25519 fallback key we need a new one.
+        const unused_fallback_keys = data["org.matrix.msc2732.device_unused_fallback_keys"];
         this.opts.crypto.setNeedsNewFallback(
-            data.device_unused_fallback_keys instanceof Array &&
-            !data.device_unused_fallback_keys.includes("signed_curve25519"),
+            unused_fallback_keys instanceof Array &&
+            !unused_fallback_keys.includes("signed_curve25519"),
         );
     }
 };