My understanding is that they mostly haven’t, with a couple exceptions like a few ISPs offering to priorities to pings for gaming (as FeelThePower mentioned), throttle certain protocols (e.g. Torrenting), or refuse to carry traffic for certain sites (e.g. Kiwi Farms). All of this would be prevented under net neutrality.

As far as I’m aware though, an extremely overwhelmingly portion of traffic (like you’d have to do a lot of digging to find an example otherwise) already adheres to net neutrality since it’s pretty pointless for a company to spend resources and goodwill to mess with traffic.

I don’t think too much will change. It is nice in the sense it will prevent an ISP from doing things against specific sites, although like mentioned above most of the protections are theoretical ATM.