It's worth noting that you are completely wrong: not only do they keep such a record, they also provide a handy chart of it on the latest prediction page so you can easily see exactly how the prediction changed over time.
- Web: https://projects.fivethirtyeight.com/2018-midterm-election-f...
- JSON: https://projects.fivethirtyeight.com/2018-midterm-election-f...
You are correct they do not publish the exact math of the forecasts, but you can see how the forecast changed over time.
Edit: For example, here are the headers on the file named presidential_elections.csv
year,office,state,district,election_date,forecast_date,forecast_type,party,candidate,projected_voteshare,actual_voteshare,probwin,probwin_outcome
These are the actual forecasts they made for presidential elections going all the way back to 2008.
[1] https://projects.fivethirtyeight.com/2016-election-forecast/...
It has not changed. They did this the whole way back when it was an independent blog before the NY Times (later ESPN then ABC News) picked them up.
They are literally charted on the page with the current prediction, e.g., for the 2016 Presidential race (which is locked on the last forecast):
https://projects.fivethirtyeight.com/2016-election-forecast/ , under “How the forecast has changed”, it charts the forecast (by default, the win probability) for the entire time the race was being forecast.
538 also predicted lower chances of victory in the weeks and months leading up to that, then adjusted it on election day vanishing away their earlier guesses. This isn't wrong on the face of it, but people infer from the final datapoint to the conclusion later that same day about 538's accuracy.
What I am saying is: 538 needs to keep a historical record of their predictions.