Are you seriously complaining about a compatibility breakage that conforms to their compatibility policy? You knew exactly what kind of guarantees you signed up for. You can't expect developers to never break anything unless you don't want any form of innovation. And as far as I know the old <% form_for %> still works! You don't have to use <%=.
> Are you seriously complaining about a compatibility breakage that conforms to their compatibility policy?
Yes, I am. Just because they have a policy in place doesn't mean that I lose my right to air my opinions.
The point with the <% thing is that it feels a bit too much like gratuitous code churn to me. <%= is what you use to output something, and you do want to output a form, so <%= is what they should have stuck with from the beginning.
The inability to use <%= for form_for was a limitation in ERB. Using <%= for form_for to make things consistent is a new feature since Rails 3. This has got nothing to do with compatibility.
The tag thing in Rails has happened over time, not in the same release cycle, but still it's annoying.