Scilab Error: Need to install legacy Java SE 6 runtime
I tried downloading Scilab and it runs fine on my Catalina, where I've done the steps below to get another Java application to run.

Download and install a recent version of the Oracle JDK, like Java SE 13 from https://www.oracle.com/java/technologies/javase-downloads.html#JDK13

Then open Terminal (it's in Applications/Utilities) and write

cd /Library/Java/JavaVirtualMachines/


and press Enter. This changes the working directory to where the Java Virtual Machines are installed. Then do

ls 


to get a list of the installed versions. Enter the Contents directory of the one you're using by writing something like

cd jdk-13.0.2.jdk/Contents/


Replace the version numbers if you have a different Java version installed.

Open Info.plist in any text editor (if you want to do that from Finder instead of Terminal, write open . to open the current folder) and search for these lines:

&lt;key&gt;JVMCapabilities&lt;/key&gt;
 &lt;array&gt;
  &lt;string&gt;CommandLine&lt;/string&gt;
 &lt;/array&gt;


Replace those with these lines

&lt;key&gt;JVMCapabilities&lt;/key&gt;
 &lt;array&gt;
  &lt;string&gt;JNI&lt;/string&gt;
  &lt;string&gt;BundledApp&lt;/string&gt;
  &lt;string&gt;WebStart&lt;/string&gt;
  &lt;string&gt;Applets&lt;/string&gt;
  &lt;string&gt;CommandLine&lt;/string&gt;
 &lt;/array&gt;


and save the file. Then try opening Scilab again. 

Sources: https://crunchify.com/os-x-mavericks-eclipse-java-issue/
and https://www.awareim.com/forum/viewtopic.php?p=48828#p48828