If it’s something in the code, that’s where I use comments. It’s the only place people have a chance of seeing it. Even when I add these comments some people ask me about the code instead of reading them. This isn’t just for others, I forget as well. Something to the effect of…
# This previously used ${old-solution}, but has moved to ${new-solution} because ${reason}
Or
# This is ugly and doesn’t make sense, but ${clean-logocal-way} doesn’t work due to ${reason}. If you change ${x} it will break.
Or
# This was a requirement from ${person} on ${date}. We want to remove this, but will need to wait until ${person} no longer needs it or leaves the company.
# This previously used ${old-solution}, but has moved to ${new-solution} because ${reason}
Or
# This is ugly and doesn’t make sense, but ${clean-logocal-way} doesn’t work due to ${reason}. If you change ${x} it will break.
Or
# This was a requirement from ${person} on ${date}. We want to remove this, but will need to wait until ${person} no longer needs it or leaves the company.