Before the maintainers head off for vacations (risks of travel and all that), there is a release fixing some errata from the previous one at https://github.com/networkupstools/nut/releases/tag/v2.8.3 (so packagers have easier time having those fixes applied right in the tarball they use), and delivering a number of updates to NUT drivers - both brand-new and old. More streamlining of ECO mode terminology and workflow.
A number of code hardening fixes planned for this release have landed (and did cause exposure and fix of certain coding issues), but many were deferred to later releases so as to minimize the potential-breakage scope in this one that aimed to be a quick small increment after 2.8.3.
For more details, please see https://github.com/networkupstools/nut/blob/v2.8.4/UPGRADING.adoc and https://github.com/networkupstools/nut/blob/v2.8.4/NEWS.adoc