Other improvements (of which there are plenty) aside, darktable-3 features a considerably better and more customisable UI. Let's not keep the users waiting!
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amd64 stable
Turns out darktable-3 only supports 64-bit architectures (or to be precise amd64, arm64 and ppc64; see src/is_supported_platform.h). Withdrawing x86 stablereq, meaning we are done here.