Adding a stored generated column is only implemented by the inefficient ALGORITHM=COPY. InnoDB should allow it with ALGORITHM=INPLACE, possibly even with ALGORITHM=NOCOPY (instantly add the column, then update the rows with the computed values). This might even be possible with LOCK=NONE.
MDEV-16329 would allow any ALTER TABLE with ALGORITHM=COPY and LOCK=NONE. The added value of an InnoDB-specific implementation would be that the table does not have to be copied.