Snap up a copy on Amazon, Best Buy, Deep Discount, CD Universe, DVD Empire, Walmart, Barnes & Noble, etc, if you don't have a copy already. Don't risk having to buy later on eBay at higher prices!
However, only the DVD set is out-of-print. Shout apparently still has the digital distribution, streaming and download rights. You can still buy digital copies of the episodes on Amazon Prime video. That's because you don't get the documentary when you buy digital copies of the episode.
IMHO, I think this has to do with the Susan Hart settlement Shout made regarding the AIP documentary on the set. Remember in that Reddit thread the mention of how Shout wanted an additional two years for physical distribution rights on the set's documentary? I think this relates to that. Hart file suit against Shout in late 2018, set goes out-of-print at beginning of January, 2021, exactly two years. Since Shout had it in print for five years since December 2015, I don't think it's a priority to re-issue the DVD.