I just tossed together a quick foldback circuit. I set up \$R_1=1\:\Omega\$ so that the foldback should start somewhere after \$650\:\text{mA}\$. I set \$R_3\$ to provide enough base current for \$Q_2\$ (\$20\:\text{mA}\$ at a rough guess) to handle that maximum load current goal. \$R_2\$ and \$R_4\$ are set for the foldback bits. (Deferring the explanation unless/until someone cares.)
The circuit goal is to start seeing some foldback when the load current exceeds \$650\:\text{mA}\$ and before it gets above \$800\:\text{mA}\$, at a guess. Honestly, I'm just throwing mud at the wall here, but hopefully close enough.
The teaching goal is to just show how to use the .OP card to plot foldback behavior:
And that's it! The x-axis only goes from smaller to larger, so I can't flip the chart without modifying the x-axis. But the x-axis is the load resistance. Clearly the supply voltage is somewhere near \$15\:\text{V}\$, but a little less because of the circuit overhead.
As can be seen, it does exhibit some foldback right at \$750\:\text{mA}\$ peak. Read the chart from right to left (lower loading towards higher loading) and you can see that the supply voltage is dead-flat until the foldback point is reached (green curve.) The red curve shows the load current increasing until the foldback point is hit, when it is forced to decline as the load increases still further.
(I could add a few more BJTs to bring \$V_O\$ under voltage regulation, while including the foldback circuit. This is old-school stuff, pre-swicher days, so it almost flows out from my fingertips without thinking much about it. But I tried to keep the above focused on the foldback part. So there is a very small change in \$V_O\$ as the load increases prior to foldback due to the increasing overhead of the circuit. Just wanted to point out that this isn't a necessary consequence and that it is possible to include both precision voltage regulation as well as current foldback. But that it requires another few BJTs to get there and I avoided it, here, for now. It would have added additional unwanted distractions. If interested, the book I cut my eye-teeth on, is Switching and Linear Power Supply, Power Converter Design, by Abraham I. Pressman, 1977. It covers this and much more from a perspective during the very slow transition period between linear power supply design and the newly developing switcher design. It was top-notch at the time. And very much worth a review if interested in how things looked back then as compared to now. I owe the author much I cannot ever pay back.)