Question: I know semantic tags were added in HTML5.

So isn't the table tag in HTML4 not a semantic tag?

Can only tags such as header, section, article, etc. added in HTML5 be called semantic tags?

I'm really curious.

Answer: 
I wouldn’t say that there were no semantic elements in HTML 4 and before. It’s more that they were mixed with elements that control visual presentation, like `<font>`.

Also `<h1>` and other heading elements were already around, that are clearly semantic. Most notably the `<form>` related tags are purely semantic, and as you say, the `<table>`.

After 1996, with CSS, *separation of concerns* between visual presentation and document structure became achievable, hence the push towards more semantics in HTML 5 and against presentation-tags.

The [Accessibility Tree](https://developer.mozilla.org/en-US/docs/Glossary/Accessibility_tree) is another proof that HTML 4 already had semantics.

It is generated from HTML (from the DOM), and serves exclusively for conveying document and interaction semantics to users of assistive technology. [The first accessibility guidelines were from the early 2000s](https://www.html5accessibility.com/browser-acc.html), so they were based on HTML 4.
> 
> I know semantic tags were added in HTML5.
> 
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HTML 5 did add new semantic elements

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> So isn't the table tag in HTML4 not a semantic tag?
> 
> 
> 

Incorrect. It is a semantic element.

> 
> Can only tags such as header, section, article, etc. added in HTML5 be called semantic tags?
> 
> 
> 

No.

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Semantic elements have *always* been part of HTML.

HTML 4 was notable in that it did a lot of work cleaning up the semantic features of the language (e.g. deprecating `font` and `center`).