I was in a similar situation at the end of last year losing all WhatsApp chats dating back to ~2016.

I got a new iPhone 15 Pro (and didn't keep it, but that's another story), wanting to upgrade from my 13 Pro.

Now normally, I just transfer all data from the old phone to the new phone which always worked fine. But WhatsApp has its own backup mechanism, which normally works in addition to the regular iCloud backup. But a year ago or two they introduced end-to-end encrypted backups, which I enabled. Apparently this blacklists WhatsApp chats from being backed up to iCloud as part of the standard iPhone backup (even when WhatsApp is explicitly enabled in the backup settings) and also blacklists them from being transferred via direct phone transfer for some reason only some developer at Meta knows.

So, that meant the new chats weren't on the new iPhone 15, but I didn't know that yet. I then launched WhatsApp on the iPhone 15, going through verifying my phone number again, which is the normal procedure. It then asked me to generate a new encryption phrase, and this should have been the first sign something went wrong. Basic WhatsApp configuration apparently transferred just fine, which is why WhatsApp never asked to restore the backup it made. This was my first failure, as I didn't suspect anything and simply set a new encryption phrase. Sure enough, WhatsApp wiped the existing (WhatsApp specific) backup off of iCloud and started a new (empty) backup. Apparently WhatsApp is designed to only support one backup per iCloud account/phone number.

The "old" iPhone conveniently asked if I wanted to factory reset/wipe it after the transfer was successful. This was my second failure, as I simply confirmed the prompt, thinking the transfer was completed (which it was, except for the WhatsApp chats) and also thinking I had the iCloud backup of the phone (which I had, but that excluded the chats as well).

My third failure then was not having any local backup of the phone. I thought I had a backup that was at most a few weeks old on my Mac, but apparently I deleted it and I also moved to a new NAS recently and didn't transfer the Time Machine backup of the Mac, so it wasn't in that backup either (I wiped the old NAS).

So it was a chain of UX nightmares/stupid design on Meta's part and my own stupidity on multiple occasions.

I somewhat cared that my chats were gone as they were a decent database for actually useful stuff and WhatsApp is searchable quite fast, so I frequently used it to find older stuff like links, photos or documents. There were some more personal chats that I'd have like to keep as well. I actually got some chat history back by asking one guy I share a lot of groups with to export their shared group chats and send them to me. I also got a more personal chat back from a person I'm close with.

Most chats are still missing and gone for good, but I'm mostly over that as I shouldn't live in the past anyway. I also tried contacting Apple to restore iCloud to an older state, but as the WhatsApp backup isn't on the actual user-facing "iCloud Drive" there's no way to recover this data.

What bothered me more back then and sometimes still bothers me is how my own mistakes contributed to losing all my chat history. Just not fucking up one single part of this would've resulted in me having a working backup of all chats/a working live version on the old phone. It was completely unnecessary to instantly wipe the old phone for example. I absolutely hate fucking something up that I'm not able to fix/do anything about.

One thing I can tell you is to focus on what lessons you take away from this. What I took from this is to of course be more careful, but also to not trust proprietary/cloud-based backups. In the back of my head I always wanted to backup my iCloud Photo Library locally to my NAS, but I never did it. I searched for an app that automatically backs up original versions of all photos/videos to my Synology NAS and I now have a regular, automated, append-only backup of all my iCloud photos. I backup personal data from my NAS to an external SSD weekly, and have a separate cloud backup of the most important data running every night.

Then there's also time. Time lets you get past most things. Sure, you'll probably think back once in a while and think "oh damn", but then you'll move on the next second and it'll be fine. Trust me.