Lots of countries have some level of exit tax on unrealised capital gains, but the devil is in the details. A painful part in the German case is that they have a fixed company valuation method that might make it untenable to leave the country.

It's one thing to tax people on assets they actually have or that are easily realisable like ETFs, as they then pay a portion of money they have ready access to. It's quite a different thing to invent a value for something and tax on that. The company ownership in question might not be realisable at anything close to that amount, especially for a startup, if you don't leave before making a profit

So don't do startups in Germany. The exit tax is just one of many reasons for that, the whole German system is bureocratic and inflexible compared to nearby countries.