Old projects and the free-software community

Isn't this a special case of the more general question about backwards compatibility, and binary compatibility? Old Win32 executables from as long ago as 1994 work on current Windows (and on 32-bit systems, even older 16-bit executables and DOS programs usually still work). If an old program goes into maintenance mode, then it should be possible to freeze a known binary image and also freeze the build environment (which in turn could be based on frozen binaries of old compilers, etc). But this is so far from current Linux practice that I doubt it will ever happen. Although there are current efforts to provide a stable binary environment with 'apps', most of the programs which count as 'old projects' are command-line tools or infrastructure rather than 'apps', and may not fit the packaging framework so well.

Then again a frozen binary image, even if packaged with some ancient build system, is definitely a second-best option. Much better to keep the old code building on current systems. As you say, this can become a chore, but that is one reason why Linux distributions exist, and a reason to prefer software which is packaged and maintained by your distribution. They will do the work of making sure it stays building, applying local patches if necessary if there is no upstream any more.