Now, all large sensors (larger than APS-H) are stitched except possibly Canon's as Canon seem to have a larger format stepper.
This internet myth refuses to die.
1. Canon has said repeatedly that it stitches its 36x24mm sensors, and that the 1D sensor size of about 19x28mm is about the largest that can be made without stitching, and this fits with the known maximum field size of 26x33mm for all suitable
steppers, and all steppers introduced for some years now.
2. Canon makes one larger format stepper about 50x50mm field size, but it is a quite old model of very low resolution, 0.5 micron minimum feature size. Rather clearly, that minimum feature size is too large for good modern CMOS pixels of about 6 microns: CMOS sensors are typically made on steppers with minimum feature size 1/15th to 1/30th of pixel pitch.
3. That 50x50mm format stepper is likely used by Kodak to make its largest sensors (for X-rays and such) which are of exactly that size ... but those have huge 24 micron pixel pitch. And since Canon sells that stepper to al comers, and for example Kodak seems to have access to one, there is no Canon exclusive on it: any major sensor maker that wants one can probably get one.
4. Nikon used to sell a stepper with field size large enough for 36x24mm sensors, but discontinued it a few years ago. I am rather sure that Nikon or Sony could get one of those if needed (sensors are typically made on older equipment), so again the claim of a Canon exclusive fails.
P.S. The joins visible on some Nikon sensors are in the toppings (AA filter), not the sensor chip itself.