People are abusing spawn protection by jumping down as they respawn so they will have spawn protection in the jail for a few seconds. Most maps this is not an issue but on maps with small jails like forbidden ways it can be abused pretty badly. I can see shorter spawn protection being an issue on maps where the spawn can easily be camped, like surf rainbow but honestly I think maps like rainbow and 10X, where one person can deny an entire team from leaving spawn should just be removed or reworked. a possible solution may be to limit spawn protection to only work in the spawn area, but I am not sure if it is possible, as I don't know too much about making plugins for tf2.
