src/vs/platform/userDataSync/common/ignoredExtensions.ts/IgnoredExtensionsManagementService/updateSynchronizedExtensions
class IgnoredExtensionsManagementService:	updateSynchronizedExtensions(extensionId: string, sync: boolean): Promise<void> {
		// first remove the extension completely from ignored extensions
		let currentValue = [...this.configurationService.getValue<string[]>('settingsSync.ignoredExtensions')].map(id => id.toLowerCase());
		currentValue = currentValue.filter(v => v !== extensionId && v !== `-${extensionId}`);

		// Add only if synced
		if (sync) {
			currentValue.push(`-${extensionId.toLowerCase()}`);
		}

		return this.configurationService.updateValue('settingsSync.ignoredExtensions', currentValue.length ? currentValue : undefined, ConfigurationTarget.USER);
	}