I just hope this tragic event improves railway safety and best practice. Modern TOCs will soon phase out this type of rolling stock, but this is still very relevant for heritage railways.
It is not common to have limited clearances on heritage railways as they often have a single track in the middle of what was previously a double track formation and have to keep vegetation away from the tracks due to the fire risk.
The Cholsey and Wallingford railway have window bars on one side of their stock but I can't think of any other preserved railway with this feature.
It does seem unlikely that preserved railways will be forced to replace their stock with Mark 4 carriages as a way of implementing sealed carriages and power doors. Whether limited opening of windows or window bars on heritage railways is likely in the future is a matter for the ORR but would seem to be a bit drastic. Charter trains may be a different matter.