Goodreads killed their developer api in December 2020. It’s obvious they’ve no intention of improving the platform. Hopefully someone can come along and provide a better experience.
Absolutely stupid that when you unlink an account the underlying data is silently lost. However due to GDPR and similar laws, irrecoverable delete is absolutely a thing. They cannot recover your data if it went through the same process of forgetting it as they would for GDPR, and there's a good chance the system wrongly treats the unlinking of Twitter as a request to forget. If they can somehow recover the data for you, they have a flawed architecture for forgetting your data, and they will not give that up as easily.
If they have a way to recover the data... they may not want to do admit they do... or put in the effort for individual users. Surely database backups exist and data is not completely lost for 6 months or whatever their backup strategy is to keep backups... GDPR or not.