Everything you need is in the new IRCv3 stuff.
There are plenty of ways to deal with join/part without join/part as join/part has never been reliable on Twitch anyway.
There shouldn’t be any reason to be using join/part. Unless you are running a greeter script and no body likes those…
I personally don’t have a problem with join/part being removed as I wasn’t using it myself. I polled the channel list from the other API as per here: Users list in IRC clients missing for me?