Re: "Recently deleted messages are not really deleted : " ... I thought when deleted messages were moved into recently deleted and you delete they would be deleted but this doesn’t seem to be the case?"
Seems, if deleted as Apple Support suggests below,
messages in Recently Deleted should be permanently deleted ... irretrievable.
Apple support's article: Delete messages and attachments in Messages on iPhone - Apple Support
Eg: says:
"Permanently remove recently deleted messages and attachments
Deleted messages and attachments remain in the Recently Deleted folder for 30 days, but you can permanently remove them before that.
- Go to the Messages app
on your iPhone. - In the Messages conversation list, do one of the following:
- Tap Edit in the top-left corner, then tap Show Recently Deleted.
- Tap Filters in the top-left corner, then tap Recently Deleted.
- If you’re in a conversation, tap
to return to the conversation list. - Select the conversations whose messages you want to delete permanently, tap Delete, then follow the onscreen instructions."