Oracle 12.1 optimize_adative_features = FALSE ; using new statistics
No, it just means that the Oracle optimizer won't use 


  adaptive features, including adaptive plan (adaptive join methods and bitmap pruning), automatic re-optimization, SQL plan directives, and adaptive distribution methods 


(quoting from the URL @JSapkota mentioned in the comments)

Oracle will still use all the information gathered via dbms_stats.gather_schema_stats -- but unless you are experiencing problems with the default value of true -- why not take advantage of those additional features?  As a side note, Oracle will, with a default installation, automatically gather statistics daily on tables that have changed enough via inserts, updates, and deletes, where "enough" is roughly 20% of the data; so you'll seldom need (or want to) manually gather statistics for an entire schema.  

You may want to gather statistics manually on a particular table or set of tables after a bulk load, etc., but then just on the affected tables, and if you don't want to wait for the daily statistics gathering to do its thing.

In short, any of the Oracle dbms_stats.gather_XXX functions will provide the needed information for the optimizer.  The only time you may want to gather statistics for an entire schema is perhaps after importing a schema into a database, after a mass update of many tables in the schema, etc.