There is a little used option innodb_defragment that would make
OPTIMIZE TABLE not rebuild the table as usual for InnoDB, but
instead cause the index B-trees to be optimized in place.

This option uses excessive locking (exclusively locking index trees).
It never covered SPATIAL INDEX or FULLTEXT INDEX. Storage space
was never reclaimed.

Because this option is not particularly useful and causes a
maintenance burden (most recently in
commit de4030e4),
it is best to deprecate it, to prepare for its removal.