That’s the reason why Warcraft 3’s customs scene was so amazing, because Blizzard didn’t take ownership of your content. That’s why there were so many custom map makers that went on to make their own games and genres. A good chunk of Warcraft 3’s charm is the fact that the game was so good and the engine so revolutionary that even a decade after its release people were still playing on it and making new games.
Me and some of my buds used to use GameRanger years ago. It worked pretty well but it had advertisements which was pretty annoying. Then there was Garena and Evolve which had some use, but I’m pretty sure Garena is either down or doesn’t support Warcraft 3 anymore. I talked to some people who are using Wc3Connect (1.30.1) and according to them there are games up with decent consistency.
I really wouldn’t worry to much about it right now. There is going to be an influx of people running older clients and playing over these services because Reforged just destroyed the community. I honestly wouldn’t be surprised to see groups of people get together and create new services for the sole purpose of playing Warcraft 3 online.