AncestryDNA is changing the way they match users, and which matches will be shown to you. Matches that share 6 to 7.9 cM will be eliminated. In addition, since AncestryDNA rounds up segments from 7.5 to 7.9999 to 8, as I understand it, some matches that appear to be 8 cm will also be eliminated. This self-answered question is intended to collect links to articles and blog posts explaining what's happening.
Roberta Estes says in her 18 July update:
ONLY segments to be salvaged will be ones in groups, with notes or matches whom you have messaged. Ancestry has confirmed that matches without these things, meaning matches in ThruLines or that you have placed in your tree will NOT BE PRESERVED unless they are grouped, have notes or you’ve messaged.
The determining factor is total cM, not smallest cM. So total cM between 6 and 7.9999, which rounds up to 8 will be removed. Multiple 6 cM segments where the total is 12 will be fine, for example. Again, it’s the total cMs, so no math needed.
The users who will be hit hardest by the change are people using small-segment matches to tease out data about distant cousins.
If you want to preserve your data, please read the blogosphere and learn how to add your matches to groups or make notes against them before the changeover ("before the beginning of August", according to Chris Paton).
My opinion: It might be a good idea to add the people you've messaged with to a group, as a safeguard, due to the fragility of the Ancestry messaging system.
Resources from AncestryDNA:
Posts from bloggers will appear in my answer in reverse chronological order. I invite community members who are using AncestryDNA to post their own answers about saving data as well. If you want to add to the list of blog posts, please put the link in the appropriate place in the calendar, so the newest posts appear at the top.