TLDR: "Mein Kampf" was never actually banned in Germany but its complicated.

I am from Germany and the fate of the book after WWII is pretty wierd, this is just written from memory and its way more complicated as history always is.

First you need to understand in nazi germany nearly everyone had a copy of "Mein Kampf" , it was given out like candy and not accepting/buying it would be pretty suspicious to the secret police. (had lots of talk about it with my 90 year old grandma, you needed to fly the swastika at every occasion and be able to produce shit like the flag and the book in case you were ever under suspicion or else... ) After the War most people got rid of the stuff or put it in the back of the attic because of ongoing denazification and to forget ( my guess is because the book is an afront to literary sensibilities).

Hitler seems to have bequeathed everything he owned to the German State, including the Copyright of "Mein Kampf". That meant the exclusive publishing rights went to the Bavarian State, because he had his official residence in Munich. Now the Bavarian Government decided to just dont print the book and nobody could legally produce and sell new copies. This worked pretty much as a defacto ban because for obvious reasons, including it's just unbelievably bad not only in content but in language as well, only (neo-) nazis or historians (who could just get it in Archives/University libraries or from that one wierd grandpa who likes showing of his medals and rants about the jews) would even want that book in post war Germany. Basically everyone was fine with the status quo and it went ignored.

Fast forward to ~2010 and Historians realise a Dilemma. They decided to start producing "Mein Kampf" as a heavily annotated "critical edition" because the German Copyright will run out at the end of 2015, and the defacto ban would be lifted. At the time that was quite controversial, discussions about banning it completely or even making it a mandatory read in history lessons, so teachers could put it in context, were ongoing.

Right after the copyright ran out the book was published, again to much controversy. I am pretty sure it actually sold well, atleast at first. Today its a nonissue again and there are still people in Germany who think its illegal to own a copy because why would you even want to read that shit, its worse than Atlas Shrugged.

Looking it up, it was never banned, it was just prevented from being reprinted due to the government holding the copyright and not making it available. It's now in public domain.

https://www.pbs.org/newshour/arts/hitlers-mein-kampf-to-go-on-sale-in-germany

The German state of Bavaria has held the copyright for Adolf Hitler’s autobiography since 1945 and has withheld publishing the book, preventing any reprints in Germany. But in 2016, the book becomes available in the public domain, which will make it widely available in Germany for the first time since World War II.

That means it was perfectly legal to have it, it just couldn't be printed in Germany.

There are Germans outside of Germany and there are also ways to access information anyway. Also, I have no knowledge of this but I'm sure there are legal ways to access it as well, for studying History or Hitler's life or whatever else. Rarely are bans total for anything. There's almost always exceptions.