Firstly, if you are not a legal professional, please temper your statements somewhat. There are very few absolutes when it comes to interpreting legalese, and what is true for one jurisdiction may not be true for all of them. (Obligatory: yes, I am aware of the existence of international copyright treaties/Berne Convention.) That said, as I denoted, I am (also?) not a lawyer. I will try to follow my own advice and not make any strong claims I am not qualified to make.

Now with that said...

> I don’t believe this is actually true. These instructions are not in the actual license terms and conditions, but rather in a distinct information section after that. In GPL-3.0, “How to Apply These Terms to Your New Programs”; in Apache-2.0, “How to apply the Apache License to your work”. My understanding is that such prescriptions and sections are not normative.

There are a few lines in Apache 2 itself which are normative and make reference to obligations regarding notices attached directly to source files. Most notably, see section 4.a[1]:

> You must cause any modified files to carry prominent notices stating that You changed the files; [...]

Of course, you can argue about what might qualify or not qualify here, e.g. maybe Git metadata is good enough, but then a tarball produced by your Git host of choice would suddenly violate the copyright license obligations.

> From a pure copyright law perspective: no, you definitely don’t need to put the license in each source file. Copyright is automatic these many years, so you don’t even need a copyright line; and license can be (and practically always is) independent of the code.

I have no idea what you mean by this. First of all, of course you don't have to put the entire copyright license into each source file; this is solely about copyright notices, which typically point to a more complete LICENSE/NOTICE file. Secondly,

> license can be (and practically always is) independent of the code.

I'm not sure what this means. Each file of a project either needs to be in the public domain or has to be covered under some kind of copyright license for anyone (aside from the original authors) to be able to distribute it. At least from a conceptual sense, the code and the copyright license are definitely not independent (This still holds with dual licensing schemes.)

> There’s even more definitely no need for a dozen lines. If you really want to put anything, one line for a copyright declaration and one line for SPDX-License-Identifier seems fair.

Some projects do basically just do this, but many of them do both. The SPDX identifier is great because it is machine-readable, and it might help you if you're ever in a situation where it's necessary.

> As for copyright lines⸺ bah, they’re such a bunch of drivel. The way people use year ranges, or just bump the year, it’s almost all such legal nonsense. As in, “if this stuff actually mattered, you’d probably have lost your copyright protection” nonsense. The fact of the matter is that copyright year stuff wasn’t designed for such easily-edited stuff as software. It was designed for “first edition copyright 1925; renewed 1935; second edition copyright 1945”, that kind of thing.

While registering copyrights or including copyright notices explicitly is not necessary to have protection under copyright law, my layperson understanding is that it does indeed afford you additional protection under law in some cases. My understanding is that since Berne Convention, pretty much anywhere on Earth anything you produce that's eligible for copyright protection does implicitly get it. However, if you ever actually wind up in court over copyright issues, the lack of clarity on what licenses go where could possibly create reasonable doubt. It's a lot of risk for what ultimately amounts to an aesthetic concern.

P.S.: Also, just so it's clear, I am mainly concerned about the copyright notices because they explicitly denote the copyright license, not because they denote the existence of copyright protection. This is especially nice to have when people contribute patches to your projects, so that it can be as explicit as possible that their contributions are under the same license as the original file.

I will stress again that I am not a legal professional, I don't study law, and at best I have spoken to people who do irregularly. However, I haven't found anyone that would disagree that it is a good idea to provide a full-fat copyright notice when possible. All else the same, it's just good hygiene at a cost of a kilobyte or so per file.

[1]: https://www.apache.org/licenses/LICENSE-2.0#redistribution