同意陈老师对what从句的分类分析。

融合型关系代词what引导的名词性关系分句是否可以作为外置主语，有不同意见。通常的解释是：名词性关系分句本质上是一个带有限制性关系分句的名词词组，所以，不能外置。关于名词词组是否可以外置，有这样的说明：

1. Rodney Huddleston在所著的The Cambridge Grammar of the English Language中，有这样的论述：

(b) Extraposition of NPs

NPs generally cannot be extraposed, but there is a very limited range of types that can:

[13]i lt's extraordinary the amount of beer he puts away.

ii lt impressed me the way she disarmed him.

iii lt's incredible the things they get up to.

The NPs concerned all have the form "the… N+relative clause". They are a special case of what we have called 'concealed questions': they are more or less equivalent to the interrogative clauses how much beer he puts away, how she disarmed him, what they get up to. At the same time, the matrix VP gives an exclamatory meaning:[i] conveys that he puts away an extraordinarily large amount of beer, and so on.Extraposition is optional: we can also have the basic versions The amount of beer he puts away is extraordinary, and so on.

此外,在The Cambridge Grammar of the English Language中，有名词性关系分句不可外置的论述：

(c) No extraposition

[6]a. What she suggests is unreasonable.

b. *lt is unreasonable what she suggests.

Like ordinary NPs, fused relatives do not occur in the extraposition construction. Here too they differ from clauses: compare That we should have to do it ourselves is unreasonable and lt is unreasonable that we should have to do it ourselves.

但是，在其他语法资料中可以找到what名词性关系分句作外置主语的实例。

1. 在Bas Aarts等所著的The Handbook of English Linguistics中，有篇Speaking and Writing English(作者Jim Miller and Andreea S. Calude)论文，在其中Can Unplanned Speech be Analyzed?一节中有这样的论述：

Many constructions begin life confined to spoken language but make their way into writing,particularly texts that are not subject to the scrutiny of teachers and publishers' editors. As an example，H&P(2002.p.1069) say that the example It is unreasonable what she suggests is incorrect, but the authors have noted the same construction，as in lt's unfair what they're doing to the union, in conversation, radio discussions,and examination scripts. Copy-editors would exclude it, but in a speech it is very common. Because spoken language is not "self-conscious" or "self-monitored"(Halliday2016 p.12), it enables the potential for innovation to flourish, making it an ideal place to look at where language change is going.

2. 在Angela Downing and Philip Locke合著的English Grammar-A University Course(Second Edition)中，有这样的论述：

30.5 extraposition of clauses

Certain types of long subject clauses are usually avoided in English because they violate the end-weight principle, and sound awkward(see p.47). Finite that-clauses,wh-nominal clauses and to-infinitive clauses can all be shifted to the end of the sentence and replaced by 'anticipatory it' in subject position. The resulting structure is called extraposition.

Extraposed clause

lt's a nuisance that the banks are closed on Saturday.

lt's horrifying what they are proposing to do.

lt would be unwise to interfere.

Extraposed clauses are much preferred in English to the non-extraposed, as they sound much less awkward. The reason for this is that they satisfy the principles of end-weight and end-focus,thus 'packaging' the information in a way that is easier to process.

3. 在Douglas Biber等合著的Longman Grammar of Spoken and Written English中，有这样的论述：

9.3.1 Structural types of Wh-clauses

Wh-clauses can be controlled by adjectival predicates as well as verbs, and they can occasionally be extraposed.

Adjectival predicate + extraposed wh-clause:

It was incredible what had happened to them.(FICT)

lt seemed incredible how much had happened.(FICT)

lt was not immediately clear how, if at all,the Soviet Leadership could enforce such a ruling.(News)

4. 在Richard Side and Guy Wellman合著的Grammar and Vocabulary for Cambridge Advanced and Proficiency中，有这样的论述：

Introductory There and It

Sometimes It refers forward to a noun clause in the sentence:

It's amazing what you learn from hanging around bars.

lt frightens me that there are so many criminals around.

we use It especially with that-clauses,wh-clauses and to-infinitive clauses that may be formal in subject position.The sentences above could be rewritten:

What you learn from hanging around bars is amazing.

That there are so many criminals around frightens me.

此外，英语母语者认为lt isn't true what they said.(They're lying)是自然的。

https://forum.wordreference.com/threads/past-tense-or-present-tense.3314817/

在葛传椝教授编著的《英语惯用法词典》中，也有what引导的名词性关系分句作外置主语的论述。

所以，what引导的名词性关系分句是否可以作为外置主语，语法学者们持不同意见。从实例来看，It is true/amazing/surprising/incredible/unbelievable what…这样的表达，比较常见。但是，如果把what引导的名词性关系分句，随便外置使用，似乎过于草率。至于网友所说的名词性关系分句作主语的那个句子，还是不太建议采用外置主语的形式。

个人观点，仅供参考。