diff --git a/packages/blocks/src/store/process-block-type.js b/packages/blocks/src/store/process-block-type.js
index 8e5b7962adfe17..98fb044f8faadf 100644
--- a/packages/blocks/src/store/process-block-type.js
+++ b/packages/blocks/src/store/process-block-type.js
@@ -71,17 +71,32 @@ function stabilizeSupports( rawSupports ) {
 		return rawSupports;
 	}
 
-	const supports = { ...rawSupports };
-	if ( supports?.typography && typeof supports.typography === 'object' ) {
-		supports.typography = Object.fromEntries(
-			Object.entries( supports.typography ).map( ( [ key, value ] ) => [
-				TYPOGRAPHY_SUPPORTS_EXPERIMENTAL_TO_STABLE[ key ] || key,
-				value,
-			] )
-		);
+	// Create a new object to avoid mutating the original. This ensures that
+	// custom block plugins that rely on immutable supports are not affected.
+	// See: https://github.com/WordPress/gutenberg/pull/66849#issuecomment-2463614281
+	const newSupports = {};
+	for ( const [ key, value ] of Object.entries( rawSupports ) ) {
+		if (
+			key === 'typography' &&
+			typeof value === 'object' &&
+			value !== null
+		) {
+			newSupports.typography = Object.fromEntries(
+				Object.entries( value ).map(
+					( [ typographyKey, typographyValue ] ) => [
+						TYPOGRAPHY_SUPPORTS_EXPERIMENTAL_TO_STABLE[
+							typographyKey
+						] || typographyKey,
+						typographyValue,
+					]
+				)
+			);
+		} else {
+			newSupports[ key ] = value;
+		}
 	}
 
-	return supports;
+	return newSupports;
 }
 
 /**
@@ -150,10 +165,12 @@ export const processBlockType =
 		if ( settings.deprecated ) {
 			settings.deprecated = settings.deprecated.map( ( deprecation ) => {
 				// Stabilize any experimental supports before applying filters.
-				deprecation.supports = stabilizeSupports(
-					deprecation.supports
-				);
-				const filteredDeprecation = // Only keep valid deprecation keys.
+				let filteredDeprecation = {
+					...deprecation,
+					supports: stabilizeSupports( deprecation.supports ),
+				};
+
+				filteredDeprecation = // Only keep valid deprecation keys.
 					applyFilters(
 						'blocks.registerBlockType',
 						// Merge deprecation keys with pre-filter settings
@@ -163,10 +180,10 @@ export const processBlockType =
 							// Omit deprecation keys here so that deprecations
 							// can opt out of specific keys like "supports".
 							...omit( blockType, DEPRECATED_ENTRY_KEYS ),
-							...deprecation,
+							...filteredDeprecation,
 						},
 						blockType.name,
-						deprecation
+						filteredDeprecation
 					);
 				// Re-stabilize any experimental supports after applying filters.
 				// This ensures that any supports updated by filters are also stabilized.
diff --git a/packages/blocks/src/store/test/process-block-type.js b/packages/blocks/src/store/test/process-block-type.js
index 3c6838e311f778..c82d08cc45d651 100644
--- a/packages/blocks/src/store/test/process-block-type.js
+++ b/packages/blocks/src/store/test/process-block-type.js
@@ -215,6 +215,12 @@ describe( 'processBlockType', () => {
 			],
 		};
 
+		// Freeze the deprecated block object and its supports so that the original is not mutated.
+		// This ensures the test covers a regression where the original object was mutated.
+		// See: https://github.com/WordPress/gutenberg/pull/63401#discussion_r1832394335.
+		Object.freeze( blockSettings.deprecated[ 0 ] );
+		Object.freeze( blockSettings.deprecated[ 0 ].supports );
+
 		const processedBlockType = processBlockType(
 			'test/block',
 			blockSettings