Surprisingly, being able to add custom config types is the most supported feature, supported by 8 libraries: BetterConfig, Carbon Config, CompleteConfig, Configurable, Configuration, MaLiLib API, OffsetUtils538, and YAML Config
After that, the one I expected to be first: automatic mod menu integration, at 6 libraries supporting it: Carbon Config, Cloth config, Configuration, MaLiLib API, McQoy, MidnightLib
Then, a three-way tie between sliders (Configurable, Configuration, oωo, and YAML Config), sections (CompleteConfig, Configuration, Fzzy Config, and MaLiLib API), and constraints (Configurable, Configuration, oωo, and YAML Config)
Bonus fact: The config library with the most extra features is a tie between Configuration and MaLiLib API:
Configuration, with custom config types, constraints, mod menu integration, and config sections.
MaLiLib API, with sliders, custom config types, mod menu integration and config sections.