Package: www.debian.org
https://www.debian.org/releases/stretch/amd64/apcs03.html.en says
a single / partition (plus swap) is probably the easiest, simplest way
to go. However, if your partition is larger than around 6GB, choose ext3
as your partition type.
OK, the installer proposed ext4, but as you wish, OK, we will choose ext3.
Ext2 partitions need periodic file system integrity checking, and this
can cause delays during booting when the partition is large.
That is nice to know but what about ext4?
In fact no need to mention any ext[234] in this whole document anymore.