From looking at the implementation of yes [0], I would say it follows closely the GNU coreutils implementation [1]. It allocates and re-uses a buffer to speed things up [2] and does not implement the old simple version [3] (or the even simpler OpenBSD [4])

[0]: https://github.com/uutils/coreutils/blob/master/src/uu/yes/s...

[1]: https://github.com/coreutils/coreutils/blob/master/src/yes.c

[2]: http://github.com//coreutils/coreutils/commit/35217221c211f3...

[3]: https://github.com/coreutils/coreutils/blob/ccbd1d7dc5189f46...

[4]: https://github.com/openbsd/src/blob/master/usr.bin/yes/yes.c