Historically, callbacks embedded in the HubSpot form embed code were the only way to trigger custom code on certain HubSpot form events (e.g. on form submission). Now, developers will be able to listen for global form events as well, allowing custom code with greater flexibility and extensibility.
What's Changing?
Nothing; this is entirely new functionality! Developers can continue to use the existing form callback functionality as before.
There aren't currently any global events related to the live chat functionality. I can totally see the use case for that, though; If you have the inclination, I would encourage you to check out the Ideas Forum on the HubSpot Community. That's where we're aggregating all product feedback/ideas, and it's the best place to make your idea visible to HubSpotters and other devs/customers. I'd also recommend linking to the idea from here so that other Dev Forum members can easily find it!