When silicone is heated, it can release volatile siloxanes into your food (source). The good news is that it’s usually tiny amounts, and it’s not as dangerous as plastic.
It’s important to note that the siloxanes released are linked to cancer. But because of the trace amounts, many experts and authorities still deem silicone to be safe and non-toxic. However, the studies aren’t long-term, so the data is inconclusive.
A great alternative is ceramic or stainless steel if you want to avoid silicone.