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Either those applications are actively maintained, or they aren't. Part of the active maintenance is to decide whether to upgrade to a new compiler toolchain version (e.g. when in doubt, "never change a running system"), old compiler toolchains won't suddenly stop working.
FWIW, trying to build a 20 or 30 year old C or C++ application in a modern compiler also isn't exactly trivial, depending on the complexity of the code base (especially when there's UB lurking in the code, or the code depends on specific compiler bugs to be present - e.g. changing anything in a project setup always comes with risks attached).