Keep in mind that the client runs on the Legion API, so there are a few semi-current addons that work without any tweaking by their creators. The site has a list of addons - including a search function - and has color-coded each one to signify whether or not it’s working yet.
The easiest one to parse through is Will It Classic. There’s a whole list of websites that are trying to compile the working WoW Classic addons. Thankfully, there are already a bunch of addons that work with the game, and addon-makers are creating new ones every day. If you’re planning on playing WoW Classic, you might feel a little naked without your addons.