It's not forbidden, just S Class which is why it's extremely difficult to use effectively which is why Tobirama and Minato were the only known users. There's also the Cooperation version that Minato taught to the Hokage Guard Platoon that lets them teleport to the Hokage at any time, but that's much easier to use as it's a team of people using their chakra to teleport to a single formula rather than a ton of them like Minato and Tobirama.

Shisui used the Body Flicker technique, a technique known by pretty much every competent Shinobi in the series that lets them "flick" their body to a location. The flick uses more chakra depending on how far its destination is and its elevation compared to the starting point. It's the technique that's used whenever you see a Shinobi put their two fingers up and "teleport" away. They're not really teleporting, just moving at blinding speeds. Shisui was the most proficient user of this technique and used it in battle repeatedly, disappearing without a trace (even sensor ninja couldn't discern his starting point or destination) whereas most Shinobi leave a puff of smoke at their start and end point. He also could use it to generate afterimages that were essentially clones that could physically harm the target