They did follow through with this once, and they subsequently announced it with "
7/18/19 - Site Outage & Removal of Old Notifications". I had a very small number of notifications at the time, but I was keeping some of them as reminders. Many of them were over a year old, so despite the fact that I was using very little server space, they were deleted anyway. I filed a trouble ticket asking for them to be restored, and my request was refused.
In summary, deleting messages based on age is not the best way to go. It would make more sense to limit the total number of messages instead.
Interestingly, looking back on that journal, they said "we removed
unread notifications which were older than a year". I don't know what they meant by that, since submissions and other regular notifications do not have a read/unread state.
As for your specific situation, you could consider saving a static copy of your notifications page as an HTML file, and then nuke your messages on the live site. You can then refer to your saved copy later.