Biggest thinga are IMO save/load for online games and also a rollback of the campaign changes from 1.33.
Though, thinking about the latter: Fucking hell, Blizzard. Fucking. Hell. It should never have come to a situation such as this in the first place!
Warcraft 3 itself never had rejoin as in you could not close WC3 start it up again login and rejoin, but if you were having connection issues it would pause the game and wait a minute or so to give you a chance to reconnect before dropping you.
Some private WC3 servers did a have better reconnection features added, like ENTConnect, but obviously those were obviously just for games hosted by their own bots (although you could still join these games from official battle.net lobby search until Blizzard nuked host botting a year or so before Reforged came out)
But if you get Desynced there’s no waiting for reconnection, you’re just dropped.
Because being Desynced means the game state on your client doesn’t match with the rest of the players, so to fix that the game would have to be rewinded until before the desync happened and no such feature existed in WC3.
And the patches leading up to reforged and Reforged itself added a ton of desync issues.
LAN doesn’t exist. Ranked Ladder & profiles didn’t exist until 2022, and it still sucks and everyone uses W3Champions instead anyway, and the auto hosted in game tournaments don’t exist.
Regarding custom games, the custom games are not made by Blizzard, they’re made by players. However in the patches leading up to Reforged Blizzard did add a lot of new functions that modders/map makers could use which were great additions, thus allowing for new cool stuff in custom games.
One super obvious and huge change was that in version 1.30 24 player support was added, instead of the usual 12.
But that doesn’t change that Blizzard then also made changes in Reforged that broke basically every existing custom map and they had to be updated to fix.
There were some desync issues and saving games in single player was broken but for custom games version 1.31.1 (the last patch before Reforged) was the best patch, you had all the new functionality added to the map editor but a lot less bugs and problems from Reforged.
Also there still haven’t even added the ability to play custom campaigns to Reforged lol. The few single player campaigns you can play in Reforged are thanks to people independent of Blizzard coming up with a work around and they’re technically just a series of custom games you play (although that is what campaigns are so).