I too just upgraded from macOS Catalina 10.15.3 to 10.15.4 and received the following message:
Legacy System Extension
Existing software on your system loaded a system extension signed by "ESET, spol. s r.o." which will be incompatible with a future version of macOS. Contact the developer for support.
This occurred with ESET Cyber Security 6.8.300.0.