When it comes to old steel off old leaf springs, hayrakes, springtooth harrows etc., the only way (imho) to determine what it can or can't be appropriately used for is to give it a go and find out. I don't think that there were standards as to what type of spring steel was used, other than that it was spring steel. I have had good luck with all of this stuff for knives, lock springs etc. and couldn;t tell you what the carbon content is in any of it other than that it's clearly high enough to be utilized for these things. You may have to experiment and adjust your hardening/tempering routine. One thing I will say that may attract some blowback is that when it comes to all of this old farm equipment or very old leaf springs etc., I harden with room temp water and not oil. Lot less messy too.
I still have huge quantities of 1084 in various sizes I bought from Admiral Steel years ago. Good steel and good seller - haven;t even checked to see if they're still around.