On: I found one of my first programs (Java, 2011) on the Wayback Machine and it runs

[brabel]

You can download the old Java build system, Apache Ant, from 2003 and it still runs without issue:

https://archive.apache.org/dist/ant/binaries/

Download the bin.zip distribution, unzip it, then run this comand:

    java -cp "lib/*" org.apache.tools.ant.Main 

It runs on Java 21 without issues :D.

Java is like Windows: it can run stuff written for it forever.