I'm sorry this page needs to exist, it's partly my fault. I created the first "large scale" SPF record (for eBay and PayPal), and agreed that this was the best way to do it. I guess the people after me agreed too, because 20 years later it's still pretty much the same shape as when I made it!

But I don't have any regrets, the format was pretty straightforward and offered a lot of flexibility. The biggest issue was the parsing libraries handling all of the "include" statements correctly.