Stash was rebranded to Bitbucket Server for the 4.0.0 major release (shipped September 2015). As part of that rebrand, bitbucket-maven-plugin was introduced to replace maven-stash-plugin and support building Bitbucket Server add-ons. The final Stash release, 3.11.6, was released December 2015.
For the 8.0 major release of AMPS, we have removed the maven-stash-plugin, and the ability to create Stash plugins, completely.
Developers will need to use the bitbucket-maven-plugin to building Bitbucket Server plugins instead.
For developers maintaining Stash plugins, they should be able to continue using AMPS 6.x versions to retain access to the maven-stash-plugin.