Hey,
I needed to delete a big amount of unused fields for a website we’re maintaining for years. As fields get renamed or deleted, meta fields stay unused in the database. While the size of the database gets unnecessary bigger we also had an issue in the search (since old fields were found).
I didn’t found a proper and safe solution without a heavy SQL query to remove those data. So I created a plugin using functions provided by ACF to remove unused metadata.
Create a database dump and give it a try:
https://wordpress.org/plugins/whatwedo-acf-cleaner/
Hope it helps and not just me
Cheers Marc
