In principle you can delete old backups. I have adopted the habit of keeping some old ones, like one week old and one month old, or so. I once realized that I had accidentally deleted a big part of my keyword hierarchy. I went back to the last month's backup and re-imported all more recent images.
I keep the last two back ups and delete anything older. Also, I regularly back up to an external drive. People sometimes overlook the fact that LR back ups are just going onto the computer hard drive. So having 1 back up or 10 on your hard drive means very little if your computer hard drive fails. I think most people have some type of remote back up, but a reminder is always helpful.