Backend API doesn't support Matrix style projects. Matrix or Multi-configuration project has build configuration where it can have multiple axis and for each combination of axis values build is repeated.
For example if there are two axis, OS with values linux and windows and JDK with values 1.7 and 1.8. then a build will result in to 4 instance of builds:
Project itself is supported and can be accessed using pipelines API but underlying matrix project properties such as axis and combination of axis is not handled.
Backend should define API model and how they are exposed via REST API.