We do not want any deleted data to be still present in the backup or del database tables. Therefore sensitive data is removed from objects before they are stored in the backup tables for synchronisation.
The table del_task_eparticipant therefore contained only lines with identifiers. Those lines do not make any sense anymore and they are not helpful for synchronisation. The update task com.openexchange.groupware.tasks.database.RemoveUs elessExternalParticipants removes these useless lines and frees used disc space.
With a later version the table del_task_eparticipant will be removed. Currently this can not be done, because this will break a possible rollback to a previous version.