[jrumbut]

I think this is about the rare cases where you have multiple competing implementations that become better or worse depending on small changes elsewhere or when a simple initial implementation can serve as documentation for a carefully optimized subsequent implementation.

I tend to think there is always a better way, say creating a module that contains both implementations as alternatives, but if this is the worst wart on the codebase it is probably a good codebase.

Just leaving giant blocks of commented out, trivial to recreate code as the system evolves is annoying. But I do think that the rule of not leaving old code in as comments is one where the bar to breaking it shouldn't be too terribly high. Personally I've gone overboard following it in the past, and all it did was hurt productivity. A few comments blocks aren't the end of the world.

[blt]

I would go for a comment like "a simpler naive implementation is in commit A0348C. It ran approx. 2.5x slower in 2019."

[jlokier]

I would do similar but include the commit timestamp and/or words from the subject to help locate the code if the commit id can't be retrieved.

Rebasing and other means of moving patches (email etc, adding signed-off-by) can cause the original commit id to become invalid, and eventually unretrievable.