Here’s my take on this.
Retrieving deleted Facebook comments is challenging because once removed from their servers, they are typically gone. However, there are a couple of avenues you can explore.
First, check Facebook’s “Download Your Information” tool. It allows you to get a copy of your data, and there’s a small chance the comment data might exist in a backup before it was fully purged. On the other hand, you could look into iOS data recovery software. These applications scan your iPhone’s internal memory for traces of deleted data. Success varies greatly and depends on whether the data has been overwritten, but it’s a potential technical solution.
A simpler, non-technical approach might be to check your email notifications. If you have comment notifications enabled, the original text might be preserved in an email from Facebook. It’s often the most reliable method.