Up to Freeciv 2.5 the experimental ruleset was a sandbox to test new game features. Starting with Freeciv 2.6 a separate …/data/sandbox ruleset is used to test new features, and the …/data/experimental ruleset is now considered to be actually playable.
In version 2.5 the experimental ruleset supported a changing vision- and city-radius depending on the city size and research, a big land unit class for catapults etc. moving only on roads or flat terrains, migration from poor cities to more desirable cities in the neighbourhood, plagues, gold instead of shield upkeep, root requirements for research, and some other new features not yet available in the classic ruleset.
In version 3.1 the experimental ruleset is not more distributed as built-in ruleset:.