Leading journals allow publishing preprints on arXiv only as there was considerable pressure. In the quantum physics community (where it was expected to post results on arXiv), a journal not allowing arXiv was not considered. So either their de facto allowed, or even started officially to do so.

I don't remember the exact dates, but Nature (who had been hesitant for a long time) gave in as well. They resisted a long time as they were (and are) considered a badge of honor.