While "solid feature" is not a formal technical term in the game's official documentation, in the context of this project and similar indie game updates, it generally refers to one of the following:
: New updates to the game often introduce or refine "solid" physical interactions between characters and the environment, which is a core mechanic of this specific genre of game.
For the latest updates or specific bug reports related to this build, you can check the developer's community page on itch.io. rar file ?