Note that the advice to meter before shifting or tilting a lens does not seem to be necessary when using a mirrorless digital camera.
The problem with metering in an SLR or even a DSLR is that the light is actually measured by a separate, dedicated metering system near the eyepiece, based on light reflected by the mirror. Tilting and shifting reportedly causes accuracy problems for this form of reflected metering.
But when using a mirrorless digital camera (or even “live view” in a DSLR) the image sensor itself is used for metering. And because the light which hits the image sensor is the light which will make up the final image, using it for metering should exactly represent how bright the final image will be.
I’ve spent a fair amount of time wandering around the streets of London capturing architectural shots on a mirrorless Canon EOS R and second-hand TS-E 24mm f/3.5 Mark II lens. And despite taking hand-held shots with amounts of shift from 0 to the maximum +12mm of shift, metering has worked absolutely fine for me even while the lens is heavily shifted. For example, this shot taken in City Of London.
I’ve also captured a number of product shot close-ups, where the lens is only two or three centimetres from the subject, and tilted almost to the maximum of 9°. Again, despite metering each shot while the lens is strongly tilted, I’ve had no problems at all. For example, this recent product shot.
In short: when metering with the image sensor itself, this advice can probably be disregarded. In fact, given that you’re metering using the image sensor itself, this old advice probably should be disregarded.