I don't think this is an issue.
I think the suggestion of
Deques a tag that the game has been free
Has the potential to just add confusion and clutter to the UI. If I want to know if the game has been free or any other data on historical pricing, I view the games historical data.
For historical low data you have the 1st Historical low which would be Free but then the next best price. So games that have been Free aren't obfuscating data.
And from what I believe @rob006 is saying, that functionality already exists on the site in the 'By Time' section.
Until right now I didn't actually know 'By Time' existed. It wasn't until I was looking at the UI more closely. @remike I would suggest adding icons to By Price and By Time to make them more visible. Example: