relative pronoun and noun phrase

[LeoBritish]

The popular assumption that language simply serves to communicate "thoughts" or "ideas" is too simplistic.

The popular assumption that language simply serves to communicate "thoughts" or "ideas": Noun phrase

and that serves here as a relative pronoun which defines the "The popular assumption", isn't it?

 

[grassy]

Yes and no: 'that' introduces a content clause there.

 

[PaulQ]

"that language simply serves to communicate "thoughts" or "ideas"" is a content clause - it informs us of the contents of the assumption, i.e. what was assumed.

Compare
His statement/opinion/claim that language simply serves to communicate "thoughts" or "ideas" ...
He thought/stated/opined/said "that language simply serves to communicate "thoughts" or "ideas" ...

"that" is a conjunction.

OED:
that, conj.
I.
1.
a. Introducing a subordinate noun clause, as subject, object, or other element of the principal clause, or as complement of a n. or adj., or in apposition to a n. therein.
The subord. clause as subject is most commonly placed after the verb and introduced by a preceding it, e.g. ‘it is certain that he was there’ = ‘that he was there, is certain’: see it pron. 4a(b). As object, it usually follows, e.g. ‘I have heard that he was there’.