Update: The German Bundestag has now revised its announcement, removing the line: "It is also planned to deactivate cost traps such as 'loot boxes' by default."
While it appears that age rating guidelines will be expanded to include descriptors for loot boxes and other so-called 'kaufenreizen', there is now no suggestion that loot boxes will be given an 18+ rating by default.
Original: On March 5, Germany's federal parliament, the Bundestag, passed a reform bill that could see games containing loot boxes given an 18+ age rating. German authorities have been looking into regulations regarding "kaufenreizen", a broad term for purchasing incentives that includes loot boxes and similar ways of encouraging players to spend additional money on games, since a 2018 study by the University of Hamburg stated that elements of gambling had become common in modern videogames.
Before the bill is passed, it will have to be ratified by the Bundesrat—a legislative body representing Germany's states—so it's not a sure thing yet. The reform bill would amend the Jugendschutzgesetz, the Protection of Young Persons Act, which regulates the sale of games and movies to minors, as well as alcohol, tobacco, gambling, and premises that offer them.
In 2008, the Act was amended to restrict videogames with excessive gore and violence, nicknamed "killerspiele", from sale to players under the age of 18, which resulted in publishers amending games for German release—swapping red blood for black, and sometimes replacing human enemies with robots, as in Carmageddon.
(On a related note, Germany's infamous ban on the use of symbols relating to unconstitutional groups in videogames, which prevented swastikas and other Nazi symbols being used, was lifted in 2018.)
A broad restriction on purchasing incentives might see games with loot boxes similarly altered for German release, or publishers might simply bite the bullet and release them as is and accept the 18+ rating. Either compromise would likely put a dent in revenues in a country that's one of the largest markets for videogames in Europe, with in-game spending up to €3.9 billion ($4.2 billion) in 2019.