In US lingo, that would be referred to as an adjustable rate, like "5/1 ARM" (fixed for five years, then adjusts each [one] year). When they say "fixed" in the US, they mean fixed for the full term.
I'm honestly surprised that became the standard, it seems like a lot of risks for the banks for what they're getting. I think it has something to do with Fannie Mae and Freddie Mac preferring to buy some mortgages and absorb the risk?
Germany has 5, 10, 15 and 20 year mortgages as the „default“. With the longer ones having a legal exit option (only for the Customer) at the 10 year mark. So in case the interest goes down, you can always refinance after 10 years. Independent of your Bank agreeing to it.
You can go longer but the bank will factor that with a higher fixed rate to offset variation.