Given the many junk answers that tend to accumulate, and the current absurdly low rep threshold for bypassing protection of questions, should questions be automatically protected? I'm not after preventing answers, but preventing spam (counting the congenial "Thanks!").
Question age is the most obvious criteria, but I'd also use view-count. The latter seems the most likely statistic to correlate with questions likely to attract junk (which the technical term for nonsense questions and comments, since it's not quite spam).
I would not want to protect questions with, for example, 100 views and 2 answers, even if one of them is accepted, because it's entirely possible a new user posting an answer there can meaningfully contribute. It might even be more likely, for that new user to have searched and found such a question – it could correspond exactly to an issue of theirs.
Numbers from thin air
As a starting point, 3-month-old questions with 500 views appears okay for automatic protection. I'll see about modifying a SEDE query for stats, unless someone beats me to it.
Starting with a sample query, 500 views may be too high and 3 months may be too old.