You can use (i.e. listen to)

You can hook in to the IActivityEvents event for ActivityCreated and when the Save button is clicked it will trigger and provide info on the changed type.

You may have to figure out some code to detect that it's a change to the ACL but here's at least the start / answer to the event question that hopefully helps

using EPiServer;
using EPiServer.Core;
using EPiServer.DataAbstraction;
using EPiServer.DataAbstraction.Activities;
using EPiServer.Framework;
using EPiServer.Framework.Initialization;
using EPiServer.ServiceLocation;

namespace TmxEpi.Ecommerce.Storefront.Site.Infrastructure.Initialization
{
    [InitializableModule]
    [ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
    public class ContentInitializationModule : IConfigurableModule
    {
        private static IContentTypeRepository _contentTypeRepository;
        private static IActivityEvents _activityEvents;

        public void Initialize(InitializationEngine context)
        {
            _activityEvents = context.Locate.Advanced.GetInstance<IActivityEvents>();
            _contentTypeRepository = context.Locate.Advanced.GetInstance<IContentTypeRepository>();

            _activityEvents.ActivityCreated += _activityEvents_ActivityCreated;
        }

        private void _activityEvents_ActivityCreated(object sender, ActivityCreatedEventArgs e)
        {
            if (e.Activity is ContentTypeActivity contentTypeActivity)
            {
                var contentType = _contentTypeRepository.Load(contentTypeActivity.ContentTypeGuid);
            }
        }
    }
}