Why 100MB ext2 boot partition recommended for linux?

Ok i think this is close to a myth specially that nowdays distros tend to update the kernels a lot during the stable cycle and not remove the old ones. Which means if you have 2 kernels (or even one) it will fill out 100MB including the grub installation and initrd. The real advice to do a /boot partition was because the bootloaders don't work well with some filesystems like xfs or btrfs.

Nowdays on modern Linux distros ext2, ext3 and ext4 are equally supported so it means you can use one of them. The use of the /boot partition makes sense if you want to use other filesystems besides ext* or you want to preserve your Kernel images between several root partitions (/). If you decide to use /boot partition i strongly advice to fill it with around 1GB.