One purpose of MRAN was to distribute Microsoft R Open, which was discontinued in 2021. MRAN has also served since 2014 as the repository of a daily archive of CRAN packages. Some R users may still be using these static CRAN snapshots as a default CRAN repository for R scripts either directly, via the checkpoint package, or via older images of the rocker/r-ver container. If this affects you, you will need to find or create a new source of the package archive before MRAN's retirement.
The blog post linked below recommends using the miniCRAN package to download and save the package archives, a process documented in detail on Microsoft Learn. Commercial users of RStudio may also wish to explore the Posit Package Manager product from the creators of RStudio.
Microsoft Community Hub: Microsoft R Application Network retirement