IMSI catchers, also known as stingrays, are fake cell phone towers that intercept your traffic and spy on you. Each generation of cell phone protocols has attempted to mitigate stingrays, but each has generally failed. With 5G, more of the traffic is encrypted in such a way that it prevents stingray spying. However, 5G networks can run in a blended 4G/5G mode, where the control channel is 4G and the data channel is 5G. This is known as 5G NR NSA (5G New Radio Non-Standalone) mode. When connected to such a blended network, a cell phone is not protected against modern stingray attacks.
The Android status bar does not differentiate between 5G modes. It will display the same 5G icon for Standalone and Non-Standalone networks. The purpose of Privacy Cell is to make it easy to tell what type of network you are connected to. If you are in an area where you usually are connected to a 5G NR SA network and Privacy Cell shows a 5G NR NSA connection, it is possible that a stingray has performed a downgrade attack to spy on your communications.
It should be noted that paltry attempts have been made to secure cell phone protocols in the past, which have never been sufficiently well designed to provide real protection. Even if you are connected to a 5G NR SA network, it is likely that there are still unknown weaknesses in the network that can be exploited by a sufficiently funded adversary. Privacy Cell will be updated regarding these types of vulnerabilities as future research identifies them.