Background ventilation on any new development is a legal requirement. Two methods of adopting this are mechanical extraction or the most popular and simplest is the use of trickle vents. All habitable rooms require 5000mm sq. of equivalent air. If you select on ordering trickle vents, the first and last door sash are fitted with trickle vents, which then automatically complies. If you do not want trickle vents fitted to the door you must ensure that other windows or doors in that room have them fitted or you adopt some other method of background ventilation.
If you are replacing an existing door and not changing the structural width of your opening, then building regulations do not apply and trickle vents are not required. However if the door you are replacing has trickle ventilation already fitted, then it is a requirement that trickle vents are fitted to the new bi-fold door.
You may choose to have a vent fitted regardless of if being a requirement, to aid background ventilation, especially in a kitchen area.