What it is and where it comes from
L-cysteine is an amino acid that softens dough. Most is made by breaking down poultry feathers or pig hair and bristles. Some is made by fermentation with microbes, which is halal and more expensive. Human hair was once used in some countries; it is not allowed in the EU, and scholars do not allow eating anything made from the human body.
Where you find it
Bread, buns, bagels, pizza bases, biscuits and pastry made by machine, and savoury meat flavourings.
What to look for on the label
On bread it may only appear as "flour treatment agent (E920)". Look for a halal logo, or a vegan label, which means the fermented kind. From poultry feathers it is accepted by certifiers only when the source is confirmed.
This is a guide, not a fatwa. Recipes change, so check the label, look for halal certification, and ask your local imam where scholars differ.