For a project like Wikipedia, stability and continuity are far more important than latest and greatest feature support; in fact, there's an argument to be made to avoid newer language features to ensure consistency, that is, if new features are used, they should be used everywhere at once. Else you end up with frankenstein code where every file could use a different style.
It helps there are automated tools for converting old style K&R function declarations to the new ANSI/ISO style, like protoize.
But K&R C is now so dated, protoize was removed from GCC 4.5 and onwards. When (in a bout of idiosyncrasy) I wanted to convert some ancient K&R C to new style a couple of years back, I ended up putting GCC 4.4 in a Docker container to make it easier: https://github.com/skissane/protoize
For most projects outside of hobby projects, I feel this should be the default case, unless there are just some huge costs savings like “improved performance by 50% and now you can save money and scale down your AWS allocations!”
