Lecithin is an emulsifier. It is found in plants such as soybeans, in egg yolks and in other animal sources. As discussed below, emulsifiers are compounds used to keep oils or fats and water dispersed in one phase (i.e., they prevent oil and water from separating).
If the lecithin is derived from plants, egg yolks or halal animals slaughtered according to Islamic law, it is Halal. Otherwise it is not. While most lecithin produced in the USA is currently derived from soybeans, it is still possible it might come from animal sources.
Unless the ingredient label says soya lecithin or vegetable lecithin, you need to check with the producer to determine the source.
And ALLAH, Subhanahu wa ta’ala, knows best.