Well this is an absolute nightmare for me. It appears i'm the only one and it's not a widespread, known or common bug.
Upgrade to my iPhone 15 Pro on Friday. I didn't pay close attention to messages at the time but noticed a favourite friend I spoke to was blank. It was until last night I realised that I could only search back 30 days on a message to a business partner I speak to a lot.
It now appears that ALL messages prior to 30 days have been deleted.
I did not choose this, I did not manually delete anything. It the oldest message is 23rd September, so it makes sense it happened when I moved to my new phone.
There is the option on each device to keep message history, "forever", "one year" or "30 days". However this doesn't apply to your entire message history in iCloud, just that device - eg if you were low on storage and didn't need 15 years of messages synced to the device.
There appears no manual way I could have achieved this. You can go into iCloud and permanently delete messages however that would delete everything, not everything prior to the last 30 days.
Every device has synced to this, Mac, iPad, iPhone. I tried to restore from a 22nd Sep backup for the iPhone 14 Pro only to find out if you have iCloud sync on for iMessages they don't even go into the iCloud backup, so that is useless for this too.
At this moment in time all messages dating back to 2008 have been permanently deleted through no choice or fault of my own and there's no system or function to restore them.
I have Apple looking into and waiting for them to get back to me, but unless they have a server side trash can that we don't see or have access to which stays full for 30-40 days like it does on my phone...I assume it's not possible.
For other users I don't know what to suggest, obviously I upgrade every year and i've never had this and no one else appears to have it - but worryingly if any glitch happens in the cloud like this - we don't have a backup anywhere on devices unless you turn off imessage syncing and then take a backup of that device (but then if you turn it back on, these messages are deleted it) it really seems you need a way to restore messages better than this!