A while ago, I removed a bunch of packages from wip where "make describe" errored out. That should be the minimum contract that a package would fulfill. If "make describe" doesn't work, the rest is also broken. Plus, I removed many obsolete Go library packages.
Other than that, there is no defined policy on removals. We remove packages that are imported into pkgsrc. Incidentally, getting the package imported is the best way to make sure it does not disappear!
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Benny