varchar2 DATA_LENGTH vs. NLS_CHARACTERSET vs. Oracle version

have 2 databases. One is 11.2.0.4, the other is 19c. (yes, it's time for upgrading to 19c *g*)

11.2 has NLS_CHARACTERSET = UTF8
19c has NLS_CHARACTERSET = AL32UTF8

On both DBs I have a table
CREATE TABLE mytable

   ( "PRESETNUMBERCONFIGPRJID" NUMBER(12,0), 


     "PRESETNUMBER" NUMBER(12,0), 

     "SHELLSIZE" VARCHAR2(1000 CHAR), 


     "FILMTYPE" VARCHAR2(1000 CHAR),


     <some other insignificant columns>
  )

On 11.2 I have an index (because of a unique constraint): CREATE UNIQUE INDEX myindex ON mytable ("SHELLSIZE", "FILMTYPE") ;

On 19c creation of the same index fails with: ORA-01450: maximum key length (6398) exceeded

Tablespace blocksize on both databases is 8k.

Querying USER_TAB_COLS with
SELECT data_type, data_length, char_col_decl_length, char_length, char_used FROM user_tab_cols
WHERE table_name = mytable
AND column_name in ('SHELLSIZE', 'FILMTYPE');

Returns
On 11.2:
DATA_TYPE DATA_LENGTH CHAR_COL_DECL_LENGTH CHAR_LENGTH C

---------- ----------- -------------------- ----------- -
VARCHAR2          3000                 3000        1000 C
VARCHAR2          3000                 3000        1000 C

And on 19c:
DATA_TYPE DATA_LENGTH CHAR_COL_DECL_LENGTH CHAR_LENGTH C