Hi Kaylee, thanks for using Ask Extension:

Discussions on this topic have been rather uncertain and limited in the past. According to Oregon State University Extension, the juglone in walnut leaves breaks down during normal composting so you can compost black walnut leaves. The composting, however, must be hot, turned and moist.  Complete composting is required (no leaf pieces) including a curing/resting phase.  Juglone naturally loses its toxicity as the leaves decompose because microbes and oxygen break it down. Cornell’s composting research shows that a warm, active compost pile speeds up this process. Once the material is fully composted, it’s safe to use in the garden.  If you are risk averse this may not be something to pursue. Check out the following links for additional information:

Walnut leaves: Safe for composting? | OSU Extension Service

Do Black Walnut Trees Have Allelopathic Effects on Other Plants? (Home Garden Series) | WSU Extension Publications | Washington State University

https://cwmi.css.cornell.edu/chapter7.pdf

I hope this is helpful.