XtraBackup 2.x vs 8.x

percona-xtrabackup-2.1.6 provided innobackupex, however xtrabackup-80 does’t have such a script anymore.Can the innobackupex still be used? Or it’s not needed? Is there any document on the upgrade and the major change?

Hello caot,
innobackupex is no longer needed. Everything is contained within the xtrabackup command. This has actually been the case ever since xtrabackup-2.4. innobackupex was just a wrapper/pointer to the xtrabackup binary. 2.1.6 is very, very, old. There is plenty of documentation on our website for the changes.
At a basic level, you can take a backup like this:

xtrabackup --backup --target-dir /var/backup --parallel 4

That will create a full backup and store it in /var/backup