The InnoDB system table column SYS_TABLES.MIX_LEN was repurposed
in InnoDB Plugin for MySQL 5.1, in
commit 91111174 (MySQL 5.1.46).
Until MySQL 5.6, it only contained a flag DICT_TF2_TEMPORARY.

MySQL 5.6 introduced a number of flags that were transient
in nature. One of these was introduced in 5.6.5, originally
called DICT_TF2_USE_TABLESPACE and later renamed to
DICT_TF2_USE_FILE_PER_TABLE. MySQL 5.7.6 introduced logic
that insists that the flag be set for any table that does not
reside in a shared tablespace, breaking upgrade from MySQL 5.5.

MariaDB does not support shared tablespaces other than the
InnoDB system tablespace. Also, some dependencies on
SYS_TABLES.MIX_LEN were removed in an earlier fix:
MDEV-13084 MariaDB 10.2 crashes on corrupted SYS_TABLES.MIX_LEN field
(commit e813fe86).

dict_check_sys_tables(): Remove a bogus debug assertion, and add a
comment that explains how DICT_TF2_USE_FILE_PER_TABLE is used.

dict_table_is_file_per_table(): Remove a bogus debug assertion.