Games banned from giveaways?

If they don't show up, you can't make em. Not sure if there is a list somewhere.

There is only a list of bundled games, but have no clue about freebies.

Well, the games that are not on the giveaway list are not allowed to be given away.
I think it's the easiest method to check there if you want to find out if one is banned /has been given away in the past

Not sure, if it's all.

Theres no official list but Zoey1 allready linked a good one.