The button has been removed. You can enable this feature in the Timeline panel's panel menu (in the upper-right corner of the panel).
I'm curious, though, why you would use this? The reason that the button was removed is that this feature is not useful for any common After Effects workflows and can instead lead to many unwanted keyframes.
It's a feature I like to use when animating text or maniupulating camera movement, and especially when working with projects similar to kinetic typography. I can definitely see how the feature can become a neusance, but I keep track of where I'm placing the keyframes and simple toggle off the button if I need to make any adjustments without keyframing.
I have to disagree, using AE everyday for the past 6 years to earn my income in a 30 year career, I have workflows that use that feature. Or perhaps I am just a disgruntled veteran.
The auto record keyframe feature is one of many that has uses that non-professional users may not see the value of. Since there is a vast array of projects, custom elements, client request, creative troubleshooting and workflows, one may be inclined to use things that others may never use. I was asked the other day on why would I use "content aware" in Photoshop since there was no practical purpose for it. I about fell out of my chair. Maybe I need an element to dance around quickly to specific marks on the screen, not random, yet frenetic. to do this the most quickly, I may turn on auto keyframe and just navigate around my timing marks, position the element and move on to the next. Granted, not used often, but it has its uses... Just saying. As a professional, if I forget to turn it off, then that is my fault and I should know better and have ways to circumvent such issues. I found this thread today looking for this button as well.
hi , go to right "timeline panel"
and click on (small Arrow) to show menu ,
then select on "enable auto-keyframe"
i hope that help you