Now we use fat on the ESP. Ext2 for boot was pretty common in the past, journaling wasn’t really needed and it was going to work with whichever bootloader you used. At the time your other partitions might use who knows what and bootloader support for that filesystem wasn’t guaranteed.
I hope Slackware 15 doesn’t still recommend ext2 for being “fast and stable” like Slackware 14 did, so at least until the start of 2022.
That reminds me that some howtos I’ve seen in the past recommended to use ext2 for a separate /boot partition.
Now we use fat on the ESP. Ext2 for boot was pretty common in the past, journaling wasn’t really needed and it was going to work with whichever bootloader you used. At the time your other partitions might use who knows what and bootloader support for that filesystem wasn’t guaranteed.