It appears this plugin has been abandoned. It's not been updated in 3 years. The Lead Maintainer (Clem Smith) has not logged into moodle.org for 3 years, and it's not possible to contact him through moodle.org's internal messaging system (https://moodle.org/message/index.php?id=1894540 returns an error).
The institution, University of Minnesota, where the maintainer (see link on plugin's github page https://github.com/at-tools) was based, no longer runs Moodle (https://vetmed.umn.edu/education/educational-support/moodle).
I have posted an issue on the massaction github page with a link to this issue - https://github.com/at-tools/moodle-block_massaction/issues/43.
From what I've seen of Moodle 4.0, it's possible this plugin should not be needed after its release. We don't use it a great deal, but some users do use it. I don't really have the resources to support this plugin, so if someone else can take it on board, I think the community would appreciate it.
