Found a solution, posting it because anybody who updates to Catalina will probably need it.
Based on the solution at https://github.com/Jackett/Jackett/issues/5589
On my system:
# cd /opt/local/bin/postgresql96
# xattr -p com.apple.quarantine plv8.so
0083;5bc65145;Safari;C16AB864-F5EB-40F8-97FF-30E58E960010
# xattr -w com.apple.quarantine "00c1;5bc65145;Safari;C16AB864-F5EB-40F8-97FF-30E58E960010" plv8.so
# xattr -p com.apple.quarantine plv8.so
00c1;5bc65145;Safari;C16AB864-F5EB-40F8-97FF-30E58E960010
Explanation:
Part of Apple’s system security now requires all executables to be signed with a valid developer certificate, and unsigned executables are quarantined (i.e., not permitted to run). Thankfully, Apple provided a CLI way to whitelist executables. The “xattr -p” command returns the current quarantine attributes of the listed executable. You’ll need to run this on your system and copy the returned string, because it will probably not match the one shown here. The “xattr -w” command writes a new quarantine attribute for the listed executable. Note the change in the first four characters of the attribute string: changing whatever’s there to “00c1” whitelists the executable so that the system will allow it to run. Run the “xattr -p” command again, to be sure your change made it into the system.