Check the end of the post, I’ve included a small tip on how to restore your Health data 
I messed up pretty bad. I didn’t back up before upgrading to iOS 27 beta. Im here because I tried to downgrade back to iOS 26.5 due to app compatibility issues. Before I did, I created a backup of my phone while on iOS 27 beta and proceeded to downgrade successfully, then I edited the info.plist correctly however after trying to restore, my backup was too large for my phone. The backup I had was 245gb and I only had around 236gb available due to the iOS and system taking up space. So I decided to try free up some space and I uploaded around 22 videos totalling to around 15gb to my MacBook perfectly fine, and deleted them from the backup using iMazing, which after a few attempts didn’t work and somehow my original backup ended up compressed. It was now 117gb and missing a manifest.db file, I’m wondering if there is a solution as I’ve tried testing iMazing and other exports but they can never find the database. Best case scenario I want to recover all my phone data, but as of now I have been trying to fix it myself for 10 hours straight to try and recover just my photos and notes. I have sentimental photos and videos, around 8k in total as well as some important things in my notes app. Those two are my main priorities so I’m wondering if anyone knows what I could do. I think I tampered with the backup too much out of frustration and now I’m lost. Thank you all
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Since the manifest.db is gone, that backup can’t be restored the normal way. What I’d suggest is extracting the data directly from the backup files to pull out your important photos and notes. I recall iMazing being able to do this.
Another option is backup repair software that attempts to regenerate manifest.db from manifest.plist. I can’t promise it’ll work, but it’s worth a shot.
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Thanks! Worked on my iPhone 13 downgrading from iOS 27 DB1 to 26.5!
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This also worked for me iphone 15 pro max downgrading from iOS 27 DB3 to 26.5. Didn’t lose anything. I waited until everything settled after recovering then i did the log out and log back in to apple account. It took like 4 attempts but all my data is my back.
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Thanks for sharing! It’s really helpful to know the downgrade worked and that you got all your data back 
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Although this problem is from June 10, I can comment on the procedure, being About restoring a backup of iOS 27 to iOS 26.5
Modifying Info.plist so that a copy of an older version appears to be compatible with a newer version is a technique not supported by Apple and does not internally convert data to the old format. Some databases or iOS 27 configurations may be incompatible with iOS 26.5.
The official solution for a copy that requires a later version consists of installing a sufficiently new version of iOS and restoring the backup there. Apple also indicates that, if a copy is corrupted or incompatible and still cannot be restored, it may not be usable and another copy will have to be used.
Therefore, if a complete copy is recovered, the safest option would be to restore it first in:
The same iPhone updated again to iOS 27 beta, or
Another iPhone compatible with that version and with enough capacity.
Then photos and notes would be exported correctly and iOS 26.5 would start from scratch. I would not try to force that modified copy directly on iOS 26.5.
My realistic evaluation
The options, from best to worst, would be:
Find a previous snapshot of iMazing with your Manifest.db.
Recover the entire folder using Time Machine/APFS.
Find the Manifest.db inside the compressed file or export.
Recover photos and videos through archive signature analysis.
Recover the Notes database through forensic analysis.
Seek professional recovery company, handing over copies and keeping the original intact.
I would no longer use the repair function on the only existing copy. I would also no longer delete videos from the backup to reduce it: iOS copies are not designed to be edited manually that way. iMazing precisely has an option to create an editable copy before modifying the file system.
The immediate priority would be to locate all versions of Manifest.db and all snapshots of the backup before that operation. That is where the greatest possibility of recovering the 8,000 photos and notes with their original structure lies. 
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