This song is/was written by singer Bob Dylan.

[In a conversation with my friends]

Me: This song is/was written by singer Bob Dylan. Should I use is or was?
Tom: You would say is if the song was currently playing, but was if you were commenting on the song after it played or just talking about the song.
Me: What if I'm just talking about it generally? Like, [XXX] is my favorite song. This song is/was written by singer Bob Dylan.
John: "Was" sounds better, but I don’t see a reason why “is” is wrong.
Tom: I’d probably go with “was”.
Me: "Is" sounds off to me because the action of writing was in fact in the past.
David: That doesn't change who the song's author is in the present, so you can still use "is". Like, you can say "this machine is broken" even though the action of breaking happened in the past.
John: The distinction lies within what you’re putting the emphasis on. If you’re referring to the song itself, “is” works. If you’re referring to the action of having written, then “was” is more appropriate.
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Question: Do you agree?

[Edit: Improved the content]

I would simply say "was".

The song was written by Bob Dylan.

Don't overcomplicate things. It hurts my head.

Bob Dylan is the author of the song. The song was written by him.

Once he dies, then he was the author of the song.

I may be wrong but I presume that you merely want to say who the performing artist is and nothing more. If so, you don't need to use the verb 'write' at all, and you need the present simple:

Who's this song by?
It's by Bob Dylan.

Do you know who this song is by?
No, I have no idea who it's by. Do you?

It doesn't matter if the person is dead or alive.

If you want to focus on the past action of writing:

Who's this song by?
This version is by Bob Dylan but it was actually written by Lead Belly.

jutfrank said:

I may be wrong but I presume that you merely want to say who the performing artist is and nothing more.

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But what if Bob Dylan not only performed a song, but also wrote it? For example, the song Forever Young.

So, it would be: "Forever Young" is a song by Bob Dylan, and it was also written by him.

Now I know that native speakers tend to use "was written" in such cases.

It was a song performed by Bob Dylan.

You could also say he wrote the song and performed it.

Tarheel said:

It was a song performed by Bob Dylan.

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I would use "is".

kadioguy said:

I would use "is".

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Why?

kadioguy said:

So, it would be: "Forever Young" is a song by Bob Dylan, and it was also written by him.

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Look at the first line of the Wikipedia entry:

"Forever Young" is a song by Bob Dylan, recorded in California in November 1973.

In the first clause, the writer used the present tense to talk about the present existence of the song. Like books, films, plays, paintings and other artworks, we talk about these things in the present tense. The second clause is a participle clause with a passive meaning. Here's a similar sentence:

"Forever Young" is a 1973 folk rock song, written by Bob Dylan.

Again, the first clause is in the present tense and the second clause is a participle clause. The difference between the clauses is that the former says something about the present existence of the song and the latter says something about its history. You can understand these participle clauses as reductions of [which was] recorded/written ... The past tense shows that you're talking about past events.

It's crucial to keep the context and the type of discourse in mind.

Without context:

"Forever Young" is a song by Bob Dylan.
"Forever Young" is a Bob Dylan song.

Both of these sentences say the same thing. The assumption is that Bob (a famous songwriter) wrote the song and therefore has legal rights to it. The sentence is not talking about the past, even though he obviously did write it in the past. It's just saying that the song is attributed to him.