The comment that I read from Elon is that he suggests 90% so that there is room for regen. The implication being don't charge to full if you don't need it, but if you know you need the full range then charge to 100%. Which is exactly what I do with my Clarity. I have no qualms about charging to full, I just don't do it when I know I don't need it. Easier for me because I am already using scheduled charging because of my time-of-use rate plan, and also my driving is somewhat predictable, i.e. I know when I will be driving more than 50 miles. Not as easy for Clarity owners that don't use scheduled charging because Honda didn't provide an option to set a lower maximum.
We don't have proof one way or the other whether charging to less than 100% SOC will have any appreciable affect on battery range degradation over the life of the car. However more than one person has reported a loss of around 10% since they have owned their Clarity, even when driving the same time of year and the same driving conditions. Confirmed by battery capacity measurements, a new battery is 55 Ah, people who have experienced range degradation get a measurement of around 49 Ah which is about 10% loss. Enough that they are somewhat disappointed that some commutes that used to be all EV now require some time on ICE even for the same time of year. 10% loss after say eight years would be understandable, however that much loss in just a couple of years is understandably disappointing to those who have experienced it. However they also have reported that so far the loss has not increased and range remains stable. Hopefully that trend continues for them.
That being said I doubt that charging to full has caused this, because as far as I can tell most people charge to full, and most people have not experienced 10% loss of range. Most likely something else is going on for these specific owners, like maybe high temperatures, or maybe the car sat on the dealer lot for several months at 0 EV miles and the battery completely discharged. But whatever the root cause, one might wonder if not charging to full (i.e. 90%) would have helped at least partially mitigate their battery degradation. No way to know unless or until there are statistics available that include how these people were charging. My guess is that charging to full makes no difference in these cases, but again we don't know.