Is it necessary for unit trust deeds to include a 'foreign exclusion' clause in relation to the stamp duty surcharge on 'foreign' purchases of residential land?

No. It is the case in all states and territories of Australia that only a unit trust with foreign unitholders would (possibly) be regarded as a foreign trust.  A unit trust that only has Australian residents as unitholders would not be regarded as a foreign trust.

Under our unit trust deed (and under unit trust deeds in general), it is only the unitholders that have an interest in the unit trust, as distributions of income and capital can only be made to the unitholders.  Therefore, a unit trust where all the unitholders are Australian residents would not be regarded as a foreign trust, although this could change if a foreign person subsequently became a unitholder.

Note that a unit trust with foreign unitholders is not necessarily a foreign trust.  E.g., in Tasmania, a trust is only a foreign trust if one or more foreign persons “have a beneficial interest of 50% or more in the capital of the estate of the trust.”

Also note that in New South Wales, the surcharge legislation specifically only applies to discretionary trusts, so a unit trust in NSW would not be a foreign trust for surcharge purposes even if it has foreign unitholders.

The position is different with discretionary trusts, as discretionary trusts generally have a much wider range of beneficiaries who can receive distributions from the trust. Read these articles for how this applies to discretionary trusts:

Victorian SRO about to take a more strict approach regarding “foreign” trusts

Deadline may be looming for NSW trusts to cease to be a “foreign trust”