Freeciv

Currently it is not possible to make a unit type unreachable yet unable to protect a whole stack, without getting rid of unreachableprotects from the entire game (game-wide as a server option.)

Unreachable can be OP exploitative in some conditions. Making a server option for it was a first step toward limiting exploits, but it throws away the ability completely for ALL game situations. As a server option only, it "fixes" things by completely removing it from EVERY aspect of the entire game.

A ruleset that is finer tuned and finer balanced may realistically portray units in different ways.
It may wish Submarines to be Unreachable by Air units, to represent they can submerge. But of course it does not want Submarines to be a magic stack-protector for an entire stack of ships. Other examples where you would want unreachable_protects ALWAYS OFF would be things like Satellites, Hot Air Balloons, Cargo Planes, AWACS, Bombers, etc.

However, the same ruleset may also have units and mechanics where it wants unreachableprotects. It is a very important tactical mechanic for representing inaccessibility and protection. Unreachable Protection is a legitimate and important tactical mechanic for some types of units.

Suggested solution:
1. Server option unreachableprotects continues to control globally if OFF.
2. If ON, individual units may have override flags for disabling unreachableprotects

This should be a simple change that solves huge tactical mechanical problems in rulesets. Right now we are forced to choose between two extremes / too much one way or too much another way.

This would allow a much improved ability to tune unit mechanics, tactics, and dynamics, over a notorious issue that is constantly putting ruleset designers into unsolvable paradoxes.