This one caught me off-guard.
Source*** Decision of the Working Group ***
Therefore, the HTML Working Group hereby adopts the 'drop the summary="" attribute' Change Proposal for ISSUE-32. Of the Change Proposals before us, this one has drawn the weaker objections.
I know I've struggled making my summaries correct in the past. Now that I know how to do it correctly, it's gone. Bleh.
(Since they removed the "5" I get more easily confused, but I believe they mean this is gone from the HTML5 spec. Which should mean, browsers/UAs will continue to support summary simply to remain backwards-compatible.)
Another thing that keeps confusing me is constant mention of using ARIA attributes to replace things like summary (and it's also been proposed as an idea to replace longdesc). One of the (important) objections to both longdesc and table summaries is that it's "hidden metadata": that is, unless you view the source, or have some extra software like a screen reader, you (either as a sighted graphical-browser-using visitor or as a document author) don't know that information is available...
but aria-describedby and other ARIA attributes have the same problem!! They're meant primarily for screen reader users, and so far I haven't seen any browsers showing ARIA attributes to me on my screen (tho Bruce might've written a plugin for Opera; wouldn't surprise me). Still "hidden metadata" isn't it?
The argument that complex tables or large tables showing specific patterns should have a "summary" out in the open (in plain text on the page) is also a good one. If users can't tell at a quick glance what's going on, you need to summarise how that table is set up and what it's saying right on the page, for all users.
But I use summary primarily for "simple" tables, as do many of us. So now what?