Disabling COMPAT_FREEBSD4/5/6/7/9 in default kernel configurations

Hi everyone,

Warner suggested that I run this by the list. In 2018, a bug report was made for disabling COMPAT_FREEBSD4/5/6/7/9 (there's no 8). 6 years later, I imagine this would be as good of a time as any to do this if there's no obvious problems doing so.

Here's the bug report: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=231768

And a pull request in the spirit of the original patch: https://github.com/freebsd/freebsd-src/pull/1228

I imagine if this sounds like a good idea, it would land in 15.0. Users could always recompile kernels with the old ABI functionality as needed. I feel like we're all a little curious if anything still uses this, and making this kind of change is probably the best way to find out.

In my opinion, if all goes well, it may be wise to remove the old code in the next major version. Could do the full list, or just FreeBSD 4 and 5 compatibility, for instance. Barring notable negative feedback, of course.

There were some concerns about Rust, but it sounds like it uses (or used?) FreeBSD 10.X features, which this patch does not remove. On that topic: https://github.com/rust-lang/rust/issues/89058

Long term, it might be a good idea to enable support for EOL-1, and maybe remove code for EOL-2, of course a less aggressive policy is also possible (EOL-2 and EOL-3?). Getting out of the single digit FreeBSD versions should be a good start, though!

Appreciate any feedback on this and hopefully we can reach some kind of consensus on how to proceed in 2024.

Thank you!

-Henrich