Wow, I had no clue r/science ended official AMAs. Thanks @simoxenham for reporting.

Well I'm glad we got to do one while they still existed. I imagine this will be a big loss for the Reddit community, as AMAs were often some of the most interesting and noteworthy posts.

From the "r/science will no longer be hosting AMAs":

However, due to changes in how posts are ranked AMA visibility dropped off a cliff. without warning or recourse. We aren't able to highlight this unique content, and readers have been largely unaware of our AMAs. We have attempted to utilize every route we could think of to promote them, but sadly nothing has worked.

It's a bit surprising to me that communities don't have more control over how content is ranked in their subreddits. It sounds to me like the main issue here is only having a single interface to interact with the underlying database. For example, if you don't like how steemit.com ranks posts, you can use a host of other Steem explorers to discover content. Not that there are great solutions yet, but hopefully Steem will avoid the issue of whole categories of content disappearing because of a ranking methodology used by a single centralized entity.