Why upper bound version numbers?

[Dominick Samperi]

Why would a package developer want to upper bound the version number
for packages like base? For example, the clash package requires

base >= 4.2 && base <= 4.3

Consequently, it refuses to install with the latest ghc provided with
the Haskell Platform (8.0.1).

Does this mean that assuming that future versions of the platform will
remain backwards compatible with prior versions is unsafe?

Thanks,
Dominick