Beside your own plugins (i.e. custom rules) or community plugins => those should be in $SONARQUBE_HOME/extensions/plugins/ there’s no need to copy (Sonar) plugins anymore,
as those are integrated in the Sonarqube edition since 8.x
Also the format of $SONARQUBE_HOME/conf/sonar.properties has changed, means it’s
no simple copy of your sonar.properties from 7.6 to 9.3, you need to edit manually.
Do not be surprised, if running on Windows as a service, there are no more
%SONARQUBE_HOME%\bin\windows-x86-64\InstallNTService.bat
%SONARQUBE_HOME%\bin\windows-x86-64\UninstallNTService.bat