[brohee]

Packages are dependant on certain version of system librairies, and binary compatibility isn't a huge concern from version to version.

[badsectoracula]

Hence the "needs at least" part, or does OpenBSD break backwards compatibility in every version for every package?

[yellowapple]

OpenBSD is particularly aggressive about breaking backward-compatibility, yes. For example: the switch to a 64-bit time_t on 32-bit systems to stay ahead of the Year 2038 problem.