While a warning was displayed for some time, latest release of Chrome disables the plugin for not following the best practices.
The Tooltivity article "Why Is My MarkDownload - Markdown Web Clipper Chrome Extension No Longer Supported?" indicates that this is caused by the plugin not using the V3 manifest format. The grace period has ended... 😔
See issue #357
The good news seems to be in Markdownload's future #342