Agree that an accurate low distortion sine wave is the best RMS source, however this is very difficult to create and quite expensive equipment. Creating a low distortion sine wave is one thing, but how do you verify RMS levels without some "reference"?
With the squarewave mentioned the verifiable "reference" is easily done with a high resolution DMM by measuring the CMOS squarewave reference VDD DC voltage. You can also measure the Average DC Waveform content of the squarewave with the same DMM, and know by design it should read VDD/2. If one is concerned about the DMM when measuring the DC waveform content of a squarewave, then a simple RC low pass works well to squash the waveform into a low frequency average moving DC term for measurement. With the DMM set to high input impedance Giga-ohm mode, then a high series R can be used.
For the True RMS AC measurement you don't want to slow down the edges but keep them crisp. Why? Because the two known waveform states are ground (zero) and VDD, both of which are easily verifiable by DMM DC measurements. Only during transition from low to high and back are the waveform characteristics uncertain and likley not precisely equal. By using a low frequency waveform not only are the squarewave odd harmonics lower in frequency, but the edges are a very small % of the waveform period, and also by using a very fast Flip-Flop with a low frequency to create the squarewave period the nearly perfect symmetry is guaranteed by design and thus contribute a very small error in the measured result.
Of course not saying this is a replacement for a proper sine wave calibration source, it's just a means to a somewhat "verifiable" waveform that is easy to create, verify and because of the waveform uniqueness has identical RMS, and Average DC values. Sinewaves can not do this nor any other waveform I'm aware of other than simple "DC". Also note that a Squarewave by definition has a "Crest Factor" of 1 and a Sinewave is 1.414!!
So rather than speculate like many tend to do, we simply verified this concept with multiple measurements with multiple quality True RMS DMMs. The Keysight KS34465A and Keithley DMM6500, both of which use computational RMS methods, and a pair of the highly trusted 34401As both of which use the analog RMS chip method.
These are our "go to" instruments at the Labs for precision waveform measurements and we will be augmenting soon with another DMM6500 and possibly another KS34465A to support the present ongoing Project where these instruments are employed.
So for now, we'll let these results speak for themselves
If you question this method, then please give it a try with your own True RMS DMMs. It's simple enough and a breadboard doesn't cost much, less than $15 total
If interested, we can send the geber files & partial BOM for the shown PCB, if I can find them
Not sure how to post them here tho, so PM.
BTW the earlier comment about the decision delay regarding the polarity under AC True RMS, a slow waveform edge speed would introduce more uncertainty and thus more potential error.
Best,