All my old messages back as far as 2009 are still there as far as I can see, but it's one of those systems where you have to scroll through and wait for more to load. I don't seem to have any left from before 2009 though.
It gives me the option to download my "1 folder and messages from our previous system ... until 31 August 2020." but that just downloads my "dealt with" folder containing 1 message. I didn't even know I had such a folder! Edit - or is it just because that was the oldest message left and so was displayed on the screen when I clicked to download?
As for whether the missing messages will come back or not, who knows?
But anyway, don't your old emails show the message text in them?