Is the AS400 still supported by IBM?

Yes. The AS400 platform is now called IBM Power Systems running IBM i, and IBM actively develops and supports it. IBM i 7.5 is the current release. IBM Power10 is the current hardware generation. IBM has committed to long-term roadmap support for IBM i, and the installed base remains large across manufacturing, distribution, and financial services. Finding IBM i support is more about finding specialized resellers with IBM i expertise than platform availability.