I can't see any problems with what ExxonMobil is doing with the "suitable for" language on the label.
The API regulations for substituting base oils and additives are complex and XOM is playing this one out "by the book". API didn't take a way the license, this was all done voluntarily by XOM.
The lack of the API donut on the label does not indicate that the formulation fails to meet the API SL/SM service grade. It just indicates that the oil has not completed the full testing sequence.
It may take weeks to get all the licensing tests completed - the above mentioned Sequence VIB runs for a total of 96 hours, the sequence VG sludge test is 216 hrs!
As far as the other major brands, substitutions do to the widespread additive shortages may be going on, we just aren't aware of it.
This is a one-of-a-kind situation & I'm sure 98% of loyal M1 users are going to continue to use the products.