Is one's boot record affected by unranked games? The wiki says:

"Practice games do not award achievements or points, and the results of practice games will not show up under the 'Ranked Games' section on profiles."

It's ambiguous regarding things like boot rate, average turn lengths, etc. Any answers? I'm afraid to test it out, since I don't want to accidentally tarnish my or somebody else's boot record.

Yes. Boots in practice games still count.