Each version of Elements is totally independent and can coexist on your computer. When you first launched 2019, you were probably given a choice to convert your existing 2018 catalog. What happens is that a new catalog is copied for 2019 and the 2018 one will be given a new name. (Something like My Catalog -1.) Each catalog can only be used by the version that it was designed for. So, currently, you are seeing the same catalog in both versions, but as soon as you make changes to one, it will no longer be in synch. IOW, they will have different contents.
If you decide to delete 2018, you no longer need the 2018 catalog. However, it is always good to have it as a backup. It can be converted again to 2019 (and presumably later versions).