Whether there's a wrapping feature depends on the implementation. coreutils base64, yes. Other implementations, maybe not. macOS's /usr/bin/base64 for instance doesn't have the concept of wrapping. FreeBSD base64, judging from the manpage, doesn't do that either. https://www.freebsd.org/cgi/man.cgi?query=base64

Edit: Oops, posted some misinformation. The mistaken part, reproduced below:

> First, wrapping by default only occurs when writing to a tty. When stdout is redirected to a file, wrapping base64 output usually doesn't make sense, so wrapping doesn't happen.

Correction: coreutils base64 wraps regardless of whether stdout is a tty.

> macOS's /usr/bin/base64 for instance doesn't have the concept of wrapping

Yes it does. "-b N" or "--break=N" inserts a line break every N characters.

Yeah. I know -w doesn't work, and missed -b when glancing at the manpage. (I use coreutils day to day.)

I ran it on macOS, and there I got:

    $ base64 /dev/urandom | head -c 1000000000 | wc
       0       1 1000000000

I haven't tried anywhere else, so it might be different on Linux or GNU or whatever.