The whole system and everything built on it that underlies trust in encryption on the modern Internet is designed in a way that requires parties called certificate authorities. That's just the design, since it was largely designed for two unrelated people to establish secure communication.

Clearly, it is not required to use a third party. First of all, you can sign your own cert using itself, then verify it manually. However, this is not the trust model that most Internet software uses. That model is closer to what SSH does, sometimes called TOFU (Trust On First Use). The model that is intended is for the certificate chain to be verified back to a trust root (ignoring other wrinkles.) There's really no particular reason why self-signed certificates must be supported.

Note that I don't think this makes the bug report invalid. It seems like a regression that is not intentional. However, the important point is that a third party still isn't needed to use the system as intended. You can, in fact, issue your own CA certificates, trust them on your devices, and then use those to sign your own certificates, making yourself the authority. This will work even on iOS as far as I know, and it follows the typical trust model so software should handle it as expected (though apps that use certificate pinning or bundle the Mozilla CA certificates statically instead of using the operating system's trust store may not work, but by and large it works.)

Personally, I just use Let's Encrypt. That way other people can establish a "secure" connection to my devices, too.

It's not being required. just that the thread is about Let's Encrypt which ostensibly easier than setting up your own CA and distributing the root certificates to your devices. Which isn't too difficult but given how many people apparently use self-signed certificates, it's a bit high a bar.

You shouldn't, and (this iOS bug aside) you don't, in general. But you're going to run into less friction if you do it the "blessed" way. That's just life.

There are many other questions to ask until you come to this question. One of them is; why iOS doesn't let me play my own mp3s?

Because you chose to use a program that doesn't accept self-signed certificates. Use a different program or a different computer that actually respects your freedom to tinker with it. Problem solved.