As far as older nurslings, like going on 2 years, and it's just 'supplimental' to a regular diet...you can use a bit more leeway.
I did a master cleanse, and several 'juice' fasts with veggies and fruits towards the end of my second childs nurse time (he was at the end of our second year nursing). I was 'protective' by making sure most of that veggie juice was green mixed with whatever I juiced and did 'sun chlorella' tablets, to ensure anything my body stirred up toxin wise, would be 'bound' and excreted, rather than circulated and end up in BM.
I did a few body wraps to help pull stuff out via my skin (a great eliminatory organ most forget about), did 'dry brushing' daily for my skin, and deep breathing (to help pull stuff out of the lungs).
I was also using water kefir (in moderation, like one small glass a day) and kombucha (1-2 small glasses a day). Other drinks were just plain water or the 'master cleanse' lemonaid. I also was doing salt water flushes via the master cleanse occasionally (even when not on the MC) and so my 'bowels were moving' fast and that helps prevent toxin re-absorbtion too!
So, if you are careful, listen to your body and watch your nursling for signs of toxins, (Breaking out, tummy irritation, irritability, etc), I think you should be fine.