I don’t want to derail this thread too far from Cartographer, as I’d like to see it make a comeback (despite how unwieldy it was to use), but in the meantime, for 2D SLAM at least, I’ve found slam_toolbox to be a well maintained and documented alternative:
Way back, my use case was for a small SBC platform with limited resources. Embedded computing power has of course improved sense 2017, but for general planar navigation, I’ve always preferred slam_toolbox to retain more headroom for CPU scheduling and other memory budgeting.