Lua: Prohibited signals
Coming more acute with #44703:
We need a way to completely stop supporting lua signals that have been deprecated for some time already.
Similar mechanisms will be needed also for the methods, and anything else in the lua API, so consider that in the design.
What makes this impossible with our current policies is that the deprecated API can be legitimately in use in a savegame formats that we support. In theory those APIs remain in savegames forever, as they are not updated even when you load the savegame to newer freeciv version and resave. But currently we support even direct loading of such scenario format versions that predate the entire deprecation warnings mechanism.
What comes into mind is a mechanism to create user defined events. Define some of them with names of deprecated and removed events and put evoking of them into new signal processors with some necessary framing code, then you can run the legacy code as if the signals still present.