The Stepper Component is not literally archived, the website your are looking is.
Google introduced a new version of material.io website in May 2018 with less components description but kept online former versions of the specifications as you can see here : https://material.io/archive/guidelines/
You can find more about this May 2018 update in the Google Design
Blog Post "Making More with Material"
All current specified components of the new update are now linked to a technical implementation. The stepper component was indeed specified but never technically implemented as part of the MD web components, take a look at this GitHub Issue from October.
Keep in mind that Google Material Design specifications and components are constantly evolving, while it seems MD started as broad guidelines, I have now the feeling that the project is really becoming a component library.