A very large number of android devices are running older versions of android. Only 12 % of total android devices are updated to lollipop. While others would not be updated to latest version due to hardware problems or manufacturer modified android running. Hence the need for supporting older versions.
2) A lot has been changed. You can check on developer.android.com. I have recently seen a change in paste method from Accessibility Service. This is just for an example. Otherwise changes are numerous.
**OCP, Java SE 6 Programmer**OCM, Java SE 6 Developer**
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