Brilliant - thanks for letting me know.
On the subject of backups - this is how paranoid I am...
I run two libraries. I pay for 200GB of iCloud, and have a "main" library holding all my recents, that is synced with iCloud and resides on my system drive. All these photos are available on all my devices. Currently I have a year of photographs there, taking up about 1/4 of my iCloud space.
I also have an archive library which holds everything older. It is on a separate drive (correctly formatted and prepared - see link below). I use powerphotos to periodically copy photos from my main library to my archive library.
I am very very rigorous about backups. Both libraries are backed up to time machine. In addition both libraries are regularly copied to a second external drive. Finally I periodically export all original images (so that I have the actual image files separately from the photos library) and copy those to a network drive backup. I have a copy of these stored away from home also (mitigate risk of fire/theft).
🤣