I don’t see anything in the pinned topics that mentions anything about stopping forum access between beta and retail launch. I would think that as long as people are following the forum guidelines, you can still post. (Discuss all things Diablo IV with fellow PC-based wanderers)
Also I did not find any reference to a freeze time in the news article which announced the Forums (Blizzard Support - Breaking News - Diablo IV Open Beta Guide)
So, I would assume we can continue to post. Provided that the rules are followed (no trolling, harrassment, etc.) Generally those poster will be dealt with by the forum mods.
Happy posting!