DECIMAL and TIMESTAMP used to have NUM_FLAG, but NEWDECIMAL was forgotten.
It's correct that TIMESTAMP does not have the flag nowadays (manual will be updated, connectors
developers will be notified).

client/mysqldump.c:
  IS_NUM_FIELD(f) removed and replaced by its definition (f>flags & NUM_FLAG).
include/mysql.h:
  - IS_NUM_FIELD() is removed because name is too close to IS_NUM() and it is not used a lot
  - INTERNAL_NUM_FIELD() is removed:
    * it forgets to test NEWDECIMAL (when IS_NUM() was updated for NEWDECIMAL we forgot
    to update INTERNAL_NUM_FIELD()), that's why client didn't mark NEWDECIMAL with NUM_FLAG (a bug).
    * it has an obsolete test for length of the TIMESTAMP field: test became accidentally wrong when length
    of TIMESTAMP was changed to always be 19 (when the format was changed from YYYYMMDDhhmmss to
    YYYY-MM-DD hh:mm:ss), never 8 or 14 anymore. That obsolete test caused TIMESTAMP to lose NUM_FLAG,
    which was an accidental but good change (see below).
    * IS_NUM() should be used instead
  - IS_NUM(f) is changed: TIMESTAMP used to be parsable as a number without quotes (when it was formatted as
  "YYYYMMDDhhmmss"); but it is not anymore (now that it is "YYYY-MM-DD hh:mm:ss"), so it should not have NUM_FLAG
  (mysqldump needs to quote TIMESTAMP values), so IS_NUM() should return false for it.
libmysqld/lib_sql.cc:
  use IS_NUM() instead of INTERNAL_NUM_FIELD()
mysql-test/r/bigint.result:
  result change: NEWDECIMAL fields now have NUM_FLAG (32768)
mysql-test/r/metadata.result:
  result change: NEWDECIMAL fields now have NUM_FLAG (32768)
mysql-test/r/mysqldump.result:
  DECIMAL columns are not quoted anymore by mysqldump. Which is ok, the parser does not need '' for them
mysql-test/r/ps_2myisam.result:
  result change: NEWDECIMAL fields now have NUM_FLAG (32768)
mysql-test/r/ps_3innodb.result:
  result change: NEWDECIMAL fields now have NUM_FLAG (32768)
mysql-test/r/ps_4heap.result:
  result change: NEWDECIMAL fields now have NUM_FLAG (32768)
mysql-test/r/ps_5merge.result:
  result change: NEWDECIMAL fields now have NUM_FLAG (32768)
mysql-test/suite/ndb/r/ps_7ndb.result:
  result change: NEWDECIMAL fields now have NUM_FLAG (32768)
mysql-test/t/metadata.test:
  test for BUG#42980
sql-common/client.c:
  use IS_NUM() instead of INTERNAL_NUM_FIELD()

Update default.conf.