spring-boot 2 supported mariadb-r2dbc though r2dbc-bom. r2dbc-bom managed dependencies was integrated into spring-boot 3 at https://github.com/spring-projects/spring-boot/issues/31702, except mariadb which didn't support r2dbc 1.0 specs.
mariadb-r2dbc now supports r2dbc 1.0 https://github.com/mariadb-corporation/mariadb-connector-r2dbc/tree/1.1.3. spring-data-relational added mariadb-r2dbc support at https://github.com/spring-projects/spring-data-relational/issues/1364.
Would be nice if spring-boot adds mariadb-r2dbc to managed dependencies.