I work for a company that manufactures grid-scale battery storage systems. I have some knowledge of this type of cell and the ancillary charging circuitry.
Most of the newer models (and I don't have the official specs on the Outie, but this is a trend across the EV-PHEV industry) have either LiFePO4 (Lithium Ferro-Phosphate) or LiNixMnyCo1-x-yO2 (Lithium-Nickel-Manganese-Cobalt Oxide, usually abbreviated NMC). NMC is preferred to LiFePO4 because they last longer and can carry a higher density of energy per mass; the main reasons LiFePO4 are still common are they use cheaper, more readily-available materials and do not spontaneously-combust.
I believe the cells in the Outie are NMC chemistry; this has been the dominant chemistry for the last two years.
Neither of these types of cell have 'charging memory', and the onboard electronics in the vehicle will prevent both over-charging and over-discharging (both of which cause irreparable damage to the pack).
These batteries work by moving the charged Lithium ions back-and-forth through the electrolyte material, as the pack charges and discharges. Eventually, the majority of the ions become immobilized at either one electrode or the other. This yields a battery that doesn't carry as much of a charge as it did when new.
It is possible to 'shock' the pack and re-mobilize the Lithium ions, but this can only be done once or twice; eventually the pack immobilizes and has to be recycled.
Finally, and most importantly, since I have a USA 2021 MY Outie PHEV [GT trim], I have not actually seen this admonition (not to use fast-charging). I must admit to being mystified as to WTF they are talking about. The electronics on-board in the car will always charge the batteries at the maximum rate the cells can drink electrons without damaging the cells, and since these batteries have no memory there's nothing to really impede their charge-discharge functionality other than age.
The only real difference w.r.t. voltage is that usually, higher voltage charging is better from the standpoint that you can use smaller-conductor cables (which weigh less) and smaller inductor hardware in the charging circuitry. Neither of which affects the battery pack's life span.
Please, anyone reading this with knowledge, feel free to enlighten me as to where I've gone wrong in my logic. I'm not Einstein nor Feynman - I'm just an engineer.