Does rebuilding an index update the last_ddl_time attribute in ALL_OBJECTS/DBA_OBJECTS?
Yes.

SQL&gt; alter session set nls_date_format='YYYY-MM-DD HH24:MI:SS';

Session altered.

SQL&gt; create table tttt1(c1 number);

Table created.

SQL&gt; create index iiii1 on tttt1(c1);

Index created.

SQL&gt; select last_ddl_time from user_objects where object_name = 'IIII1';

LAST_DDL_TIME
-------------------
2018-05-07 16:32:46

SQL&gt; alter index iiii1 rebuild;

Index altered.

SQL&gt; select last_ddl_time from user_objects where object_name = 'IIII1';

LAST_DDL_TIME
-------------------
2018-05-07 16:33:10