I've noticed a lot of sites from bigger companies put information (e.g. about page) in the footer and save the main navigation for core content.
Is there some rule of thumb for when something should go in a footer vs the main navigation?
Everything depends of the user expectation. When more and more companies doing it, more people will know that the lik is on the bottom.
But take care!
In an e-commerce, no problem, people are looking for pproducts and not about us. After decided which product will buy and which store the user will buy, they will search for about page if they think it's important. Consequently, you can put in the footer and save space to make the navigation clean and easier.
In B2B websites probably about us should be on the top because normally people enter on website to discover who they are and what they do.
As everything when we are thing in user experience you must listen and study about the user, their expectations and what is the most common between the competitor. Because if all your competitors adding about us on top, probably the user will looking for this link on top too considering that they always enter on differents website to compare products and prices.
Yes, there is a rule of thumb applied when placing most the interaction heavy CTA into the bottom bar or in the lower half of the phone. As they the easiest and most accessible area for the mobile. For Example, Uber, lift, google maps or other apps have their most interactive CTA in the lower area.