Turns out private selectors are attached to the thunk proxy!

Well that was easy. :)

packages/block-editor/src/store/private-actions.js

@@ -7,7 +7,6 @@ import { Platform } from '@wordpress/element';
  * Internal dependencies
  */
 import { blockTypePromptMessages } from '../components/block-removal-warning-modal';
-import { isRemovalPromptSupported } from './private-selectors';
 
 const castArray = ( maybeArray ) =>
 	Array.isArray( maybeArray ) ? maybeArray : [ maybeArray ];
@@ -156,12 +155,7 @@ export const privateRemoveBlocks =
 		// register using `toggleRemovalPromptSupport()`.
 		//
 		// @see https://github.com/WordPress/gutenberg/pull/51145
-		if (
-			! forceRemove &&
-			// FIXME what's the best way to unlock a private selector in this
-			// context?
-			select( ( state ) => isRemovalPromptSupported( state.root ) )
-		) {
+		if ( ! forceRemove && select.isRemovalPromptSupported() ) {
 			const blockNamesForPrompt = new Set();
 
 			// Given a list of client IDs of blocks that the user intended to

packages/block-editor/src/store/test/actions.js

@@ -614,13 +614,11 @@ describe( 'actions', () => {
 			const clientId = 'clientId';
 			const clientIds = [ clientId ];
 
-			// FIXME: Once the private action `privateRemoveBlocks` is properly
-			// unlocking the private selector `isRemovalPromptSupported`, this test
-			// should rewritten to do the right thing.
 			const select = Object.assign(
 				( selector ) =>
 					selector( { root: { isRemovalPromptSupported: false } } ),
 				{
+					isRemovalPromptSupported: () => false,
 					getBlockRootClientId: () => undefined,
 					canRemoveBlocks: () => true,
 				}
@@ -731,13 +729,11 @@ describe( 'actions', () => {
 		it( 'should dispatch REMOVE_BLOCKS action', () => {
 			const clientId = 'myclientid';
 
-			// FIXME: Once the private action `privateRemoveBlocks` is properly
-			// unlocking the private selector `isRemovalPromptSupported`, this test
-			// should rewritten to do the right thing.
 			const select = Object.assign(
 				( selector ) =>
 					selector( { root: { isRemovalPromptSupported: false } } ),
 				{
+					isRemovalPromptSupported: () => false,
 					getBlockRootClientId: () => null,
 					canRemoveBlocks: () => true,
 				}
@@ -762,13 +758,11 @@ describe( 'actions', () => {
 		it( 'should dispatch REMOVE_BLOCKS action, opting out of select previous', () => {
 			const clientId = 'myclientid';
 
-			// FIXME: Once the private action `privateRemoveBlocks` is properly
-			// unlocking the private selector `isRemovalPromptSupported`, this test
-			// should rewritten to do the right thing.
 			const select = Object.assign(
 				( selector ) =>
 					selector( { root: { isRemovalPromptSupported: false } } ),
 				{
+					isRemovalPromptSupported: () => false,
 					getBlockRootClientId: () => null,
 					canRemoveBlocks: () => true,
 				}