I don't know about cooking apples or carrots!!!
We used to buy big bags (nets) of carrots really cheaply from our feed store in the UK and slice them up every day but I've never seen them sold like that here
My Grandmother was always telling me how they used to cook potato's for the horses in the war because grain was scarce
Boiling linseed (flaxseed) I wouldn't do now you can buy it as an oil. Since you can grind it I'm not sure why it was boiled it unless it was more digestible that way, it was always told to me that linseed had to be boiled really well to remove the risks of hydrogen cyanide poisoning but it seems that this has now been found to not cause a problem but it still shouldn't be soaked before feeding
I was really pleased to see that people do still feed like this in the UK and that you can buy purpose made boilers for the job Recipes « Farvis Boilers Ltd