Just found your answer. "That" can be omitted when the meaning of the sentence remains clear. 2.
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Conjunctive that
(1) Nominal that-clause as the object of a verb
When conjunctive that-clauses are used as the object of a 'soft' verb like 'tell', 'hear', 'bel
As you know well, <so that> can mean both PURPOSE and RESULT when no comma is put before it. This ambiguity is retained even when 'that' is deleted from it