Some addons are available from other providers, although they defaulted to installing from Curse (which is why you’re having trouble right now). You will need to change your addons to update from other providers.
Wowup has a guide on migrating addons from Curse to other providers:
The Wowup discord server has been very helpful when I migrated as many addons as possibble from Curse.
It helps to enable wago.io as an addon provider as well (if you’re ok with a small ad that’s sandboxed to be as unintrusive as possible) since it has quite a few of the popular addons from Curse.
Here is another guide that helps with installing addons from other sources, including GitHub: