I haven’t found it in a search, so here goes.
The problem: The Curseforge app moved all your addons to a Classic TBC category, so now you have Classic TBC and WoW 2 on your launcher. There is no Classic Era category in the dropdown, and no searchable Era addons conveniently appear. No addons show when you load Classic Era to play WoW.
A solution: Manually copy all addon files from your Classic TBC Addons folder to your Classic Era Addons folder. Reload Curseforge. Three categories now appear: BCC - World of Warcraft, World of Warcraft 2, and Classic. Right click any addon with an exclamation point and select “Reinstall.” Categories now are searchable for new addons as desired.
This helped me with everything except Auctioneer (for TBC Classic), which apparently is a work in progress.