At the moment, for compatibility with the Java Platform Module System (JPMS), MariaDB-Connector provides an Automatic-Module-Name entry in the manifest. This suits basic usage in a modular application.
A full module descriptor, i.e. a module-info.java file, will allow full usage in a modular application. Complete module descriptors are a requirement for realizing further gains in JPMS most notably the jlink tool. Whereas Automatic-Module-Name is the bare minimum, a true module descriptor means full support.
Strictly speaking, adding a module descriptor is backwards compatible, even with JDK 8. However, there are some tools which ignore the specification and scan for module descriptors, even inside META-INF/versions/. These tools have been updated by now - the bugs surfaced around 2017-2018 - so it may not be an issue for the majority of users.
For this reason, I thought that adding a full module-info as part of 3.0.x would make the most sense - a module-info file is technically backwards compatible, but for matters of practicality, it would help users to have it introduced in the 3.x release.