The feature was never added, a deprecation message saying that it's been
removed is the closest option to the situation. While terrain.cpp itself
supports vision_alias, it was never added to the calculations in movetype.cpp,
therefore it never affected units' vision. It's also not shown in the help
about the terrain.
This doesn't affect the features of units having vision costs different to
movement costs, or of units having a different number of vision points to
movement points, both of which are still supported.
While I'd encourage anyone who wants to implement vision_alias to do so, at the
moment I feel that the still-supported features in the same area haven't been
explored yet, and that vision_alias isn't going to developed before 1.16.0 is
released.
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