Everything imported into one of the LRCC apps is automatically backed up. So once you are sure everything is sync’d to the cloud you could delete originals from your folders or archive them to an external drive whichever is preferred? This assumes you are using the system cache and you have not elected to store a local copy under preferences. If the cloud is to be your only back up you would need to take care not to delete files accidentally, although Adobe provides a warning for your confirmation.
Adobe provides a download tool which enables everything to be downloaded from the cloud back on to your computer. Additionally, Adobe will continue to store your original images for one year after your membership lapses. During that time, you can continue to launch Lightroom CC to download your original files from the cloud
If you wished to return to LR Classic you can enable sync in LR Classic and everything would get downloaded into the LR Classic catalog with the exception of keywords.
No, it is not safe to delete your originals.
Multiple sources may have copied the expression "backed up" from LRCC's marketing, but "stored" would be a better word. Having a single copy of a file does not count as backup in any meaningful sense of the word, regardless of whether it is in the cloud. If you accidentally delete that file through LRCC, it is gone forever - as is LRCC's local storage copy. If you end your subscription, the file is deleted if you don't download it within a year. So don't let loosely-worded marketing hype mislead you - only delete originals from your hard drive when you know you have your own backups.