After further digging, the solution to this problem is to place a plist file into /Library/LaunchDaemons/ which runs and sets the shared memory parameters on boot. On Catalina this has to be preceded by disabling SIP, and remounting the / file system as writable, before installing the plist file.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>shmemsetup</string>
<key>UserName</key>
<string>root</string>
<key>GroupName</key>
<string>wheel</string>
<key>ProgramArguments</key>
<array>
<string>/usr/sbin/sysctl</string>
<string>-w</string>
<string>kern.sysv.shmmax=268435456</string>
<string>kern.sysv.shmmni=128</string>
<string>kern.sysv.shmseg=32</string>
<string>kern.sysv.shmall=65536</string>
</array>
<key>KeepAlive</key>
<false/>
<key>RunAtLoad</key>
<true/>
</dict>
</plist>
Moreover, I am not sure when OSX stopped using the sysctl.conf file, but digging around my Mojave disk I found a plist file with the these same parameters; so the sysctl.conf must have been from a previous upgrade, and probably not even used under Mojave.
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