Making it optional Encourages people to think that looking-at is just
like looking-back, with the same performance characteristic,
Clarifying such things is what doc is for. (Yes, and even
sometimes byte-compiler messages.)
whereas making it mandatory encourages people to put a
reasonable limit and hence avoid pathological behavior
at the end of large buffers.
But you have _not_ made it mandatory. Have you?
And yet you issue a "warning" saying that it is mandatory
(or whatever "2-3 required" arguments is supposed to mean).
False alarm. (Circulez ; il n'y a rien a voir.)
There are many, many "best practices" that are handled
normally by documenting them, including many, many that
involve far more serious consequences than a slowdown from
leaving off a LIMIT argument for `looking-back'. Really.
Programmers are used to such notes. And even much more
important performance (and correctness and completeness)
guidelines are not blown out of proportion and dressed up
as phony "warnings".
(This "M'as-tu vu ?!!" is the equivalent of a flashy 1995
web page with loads of revolving animated GIFs. Another
distraction in a sea of distraction.)
Not a big deal in any case, on its own. But symptomatic
of a misguided approach to communicating such helpful
but non-critical info to users.
Sprinkling "**WARNING!!!**" everywhere, for even the
slightest bits of notable information, dilutes what real
warnings are about. It decreases the signal-to-noise
ratio, and just "encourages" people to ignore such
messages altogether, which is not helpful and no doubt
not what was intended.