I assume you mean "not a legal requirement" rather than "not legal"?
Some previous discussion here https://forum.safeguard.co.nz/discussion/78/signing-for-attendance-at-toolbox-meetings/p1
There are requirements for making people aware of hazards etc. A toolbox talk is one way of meeting some of these obligations, so it is a bit more complex than whether it is or is not a legal requirement.

HSWA section 36(3)(f) requires "provision of ... instruction ..." and section 61 requires "practices that provide reasonable opportunities for workers who carry out work for the person having control of a business or undertaking to participate effectively ..." I think toolbox meetings fit these requirements provided workers are able to participate (eg, Q&A).
Toolbox meetings may fit best with work that is carried out intermittently, or that requires a permit-to-work and in many situations a toolbox meeting might be the simplest way of making sure that workers are reminded of the work practices they should/should not follow in a given situation. It s not a substitute for appropriate training.
Should those present be asked to sign the minutes? That might not be practicable. The minutes might not be available to sign for a day or so and might not include all that was said. Could the meeting be recorded (audio or video)? I would certainly be reasonable to have an attendance list, especially if the work is high risk.

"Not a legal requirement" in and of themselves, however toolbox meetings (or a version thereof) are used in many workplaces as an effective means to enable front-line workers to participate in health and safety - which most certainly is a legal requirement. If someone is challenging a lack of toolbox meetings, the answer is to show them exactly how workers are enabled to take part in discussing and responding to safety and health issues on a regular basis - and if this is not happening as well as it could, there is a real question to be answered, and maybe an opportunity to improve this practice.
As an added comment, I have never found it practicable or meaningful to get anyone to "sign off" on minutes at a front-line level - simply record who was there, and a some notes on what was discussed. Important safety issues can't be "signed off and away" - they need to be addressed in practical terms on a daily basis, and maybe at more than one toolbox meeting :-)

You won't find anything that says toolbox talks are a legal requirement, because they're not. This means of course they are not a legal requirement, just one of those reasonably practicable steps an employer should consider.
Our workers find them of great benefit. We do not require each person to sign off, but the person leading the talk is required to record those attending into the booklet.

The requirement to sign off is being used more, as some prequals were requiring it at one point.

The reason for that is protection around an individual being at a meeting but maybe not being there for the entire thing. The sign off is effectively then saying I understand and accept everything that was in the meeting. It's now on them to find out what happened when they went out for that call. They can't use the excuse that they missed that bit anymore, if anything goes wrong.