At the same time, a new iteration of 5.1 was forked from the branch of 5.0, which features enhancements and other internal improvements, whilst aiming to be 'almost entirely compatible' with content designed for 5.0.x, such as themes.
A decision was made to delay some of the more signifigant changes originally intended for 5.1 to a future release, which will be known as StepMania 5.2 (see here for more details), and is in continued development on the master branch.
