This is the case with old code in general, though. 2003 Javascript, 2003 C++, any Java, especially 2003 C#, any C, 2003 Python, etc are all horrible messes to work with today. C++ got so much better with C++11/14 I can't even think of it as the same language. Python literally broke itself. Java is pretty much dead except for on Android (consumer facing) for good reason. C# was practically an infant in 2003 and got so much better with so many frameworks around it since then. C is still as bad as ever, and Rust / Go didn't exist, and Scala came out that year.
It is why frameworks and toolkits are so important. You want to minimize your codes aging surface area, and having individual parts you can manage and age independently kicks the pants off of starting from int main() and hoping for the best. Wesnoth's greatest library dependency is SDL, which honestly should make the engine fairly futureproof if they could get it ported to SDL2. But that doesn't change how everything else is pretty much raw C++, with just AI rules in Python or Lua.
I'd have to look into if they are giving a thumbs up to std=c++14 or not, because if they are I might go poke around just to see how much you can fix. I'll have to read more on that though.