End of last year, itemis retired their https://projects.itemis.de/ server for JetBrains MPS usage. Instead, they introduced the https://artifacts.itemis.cloud/ server, which has a much nicer overview of all the downloadable opensource artifacts. Usually, when I want to get IETS3 (most of it for KernelF-purposes) artifacts, I can simply download everything through the dependency tree. The most low-tech and simple way to do this is to look in https://artifacts.itemis.cloud/service/rest/repository/browse/maven-mps/org/iets3/opensource/ for whatever version I need. The file format is <mps_major_version>.<build_number>.<commit_hash>, e.g. 2023.2.6743.a25164c.
Using the .pom file that is with each of the artifacts on artifacts.itemis.cloud, you can follow the chain of dependencies, all the way down to JetBrains MPS itself. For example, 2023.2.6743.a25164c has a .pom file (opensource-2023.2.6743.a25164c.pom) that points to mpsqa 2023.2.367.4db1dc2 and mbeddr platform 2023.2.24342.cdeb54a, which can be found in https://artifacts.itemis.cloud/service/rest/repository/browse/maven-mps/org/mpsqa/all-in-one/ and https://artifacts.itemis.cloud/service/rest/repository/browse/maven-mps/com/mbeddr/platform/, respectively (I usually go to the relevant package directory and search for the build number to find the right dependency).
I hope the above describes in enough detail a replacement way for downloading artifacts from the deprecated and removed projects.itemis.de.
That being said, you can still download relevant packages from the maven repositories on github packages of each of the relevant open source projects (IETS3, mbeddr, MPS Extensions, etc.), but I don´t know how long packages are retained there.