Finding extents of Oracle BIN$ segments
Do you know since when those segments are hidden(?), and is there a way to actually find their extents?


You can find their details from *_SEGMENTS views/synonyms.

SQL&gt; select bytes from user_segments where segment_name='BIN$XYbYSYopQFTgUzIUqMA4vA==$0';

     BYTES
----------
     65536



  Will those extents still stop me from shrinking a data file (or tablespace)?


Yes, they may stop you to shrink the data files. Dropped tables and any associated objects such as indexes, constraints, nested tables, and other dependent objects are not moved, they are simply renamed with a prefix of BIN$.

Consider the following(it's just for demonstration purpose only) example.

SQL&gt; conn sys@orclpdb as sysdba
Enter password:
Connected.
SQL&gt; create tablespace tbs1 datafile size 1M;

Tablespace created.

SQL&gt; alter user jay quota unlimited on tbs1;

User altered.

SQL&gt; create table t1(id number, col1 varchar2(100)) tablespace tbs1;

Table created.

SQL&gt; insert into t1 select level, 'dsfldshfkjdhsfkdshfdklsjfhdkhfdks'||level 
     from dual connect by level&lt;=1000;

1000 rows created.

SQL&gt; commit;

Commit complete.

SQL&gt; select bytes from user_segments where segment_name='T1';

     BYTES
----------
    983040

SQL&gt; conn sys@orclpdb as sysdba
Enter password:
Connected.

SQL&gt; select ts#, name from v$tablespace;

       TS# NAME
---------- ------------------------------
        10 TBS1

1 rows selected.

SQL&gt; select bytes from v$datafile where ts#=10;

     BYTES
----------
   1048576

SQL&gt; select file# from v$datafile where ts#=10;

     FILE#
----------
        55

SQL&gt; conn jay@orclpdb
Enter password:
Connected.
SQL&gt; drop table t1;

Table dropped.

SQL&gt; select object_name from recyclebin;

OBJECT_NAME
--------------------------------------------------------------------------------
BIN$XYY3afBNKwPgUzIUqMAIkA==$0
BIN$XYYmdj0XKLjgUzIUqMB2iA==$0

SQL&gt; conn sys@orclpdb as sysdba
Enter password:
Connected.
SQL&gt; alter database datafile 55 resize 512K;
alter database datafile 55 resize 512K
*
ERROR at line 1:
ORA-03297: file contains used data beyond requested RESIZE value

SQL&gt; conn jay@orclpdb
Enter password:
Connected.
SQL&gt; purge recyclebin;

Recyclebin purged.

SQL&gt; conn sys@orclpdb as sysdba
Enter password:
Connected.
SQL&gt; alter database datafile 55 resize 512K;

Database altered.



  Hm, on my 12.2 system I see no BIN$ segments in *_SEGMENTS views (with SYS as SYSDBA user). And I think they never showed up in *_EXTENTS, or?


There is no recycle bin for SYSDBA privilege.


  Each user can be thought of as having his own recycle bin, since unless a user has the SYSDBA privilege, the only objects that the user has access to in the recycle bin are those that the user owns. 
  What is Recycle Bin?