on the datasource(s) there is a TAB for Types.

For each type, there is a set of properties.

you need to set the 'Delete items' property to true for the side on and type for which you want the deletes to propagate.

i strongly recommend carefully reading through the online help

https://admhelp.microfocus.com/connect/

MF Connect is a server centric product. There is far more to it than meets the eye - or can be gleaned from the UI.

to clarify this further...

I thought you meant that things already synced from ALM/QC to Octane should then be subsequently deleted in Octane on a later sync.
To achieve this, you would need to set the 'delete items' flag on the Octane Type which is mapped to the alm/qc requirement type in MF Connect.
In this context, something deleted on the one side will be deleted on the other side, provided the 2 were synced to each other historically (prior to the deletion)

however,
"Deleted requirements in ALM are not sync'ed to Octane"
MF Connect does not "sync" deleted items.
If some requirement is deleted in alm/qc, and was never related to any equivalent artifact in Octane,

then that requirement will not be then synced over to Octane (either in a deleted or undeleted state)
That concept does not exist.