This IBM-supplied IBM i was available to to do the IBM-supplied open labs in order to learn RDi. But it no longer is reachable through my 5250 emulator. What happened to it?

Yes embox1.demos.ibm.com has shut down and due to the generosity of Larry Bolhuis and Pete Massiello, we are now hosting the demo on common1.frankeni.com. You still use http://ibm.biz/rdi_labs_getuserid to get an assigned id (you need to authenticate with an IBM ID which is free to create). All of the PDF for the labs have been refreshed at http://ibm.biz/rdi_labs

Note that Lab 4 has new content on conditional Service Entry Breakpoints!

More info can be found on this blog post

https://community.ibm.com/community/user/power/blogs/edmund-reinhardt1/2021/02/27/rdi-labs-update