Reproducibility is a second order effect. It's not a direct goal (at least originally) and as already discussed in this thread, more or less to death, not something that is required. It is very easy to write a derivation that cannot be reproduced. Sometimes it's even useful. Purity isn't really required at all. I wouldn't even say reproducibility is encouraged. It just sort of falls out of the system.

If Nix was worried about impurities that impact reproducible builds, they certainly wouldn't make live timestamps readily available in the pure environment, would they?