It's really not a change or any of those. Mail services have behaved this way in some form or way for a while, these specs have worked like this from the start and the policy is only a logical conclusion of it all.

Untrusted forwarders (also sometimes known as open relays) have been heavily frowned upon for a really long time. SPF has never worked for forwarders that don't do rewriting. Those letters have always been negatively looked upon, unauthenticated mail is a remnant of times long gone.

Google's policies about authentication are the bare minimum you should be doing anyways if you care even a little bit about your customers or recipients in general.