100% is not so absolute a specific line in the sand as it sounds.
I own a 2023 model 3 RWD and obediently charge it to 100% by Wall Connector weekly.
I also live at a location where my most common driving involves descending a gently sloped hill on a major road which loses me about 1000 feet in about 4 miles, after first spending about a half mile on local minor road.
I own and routinely use Scan my Tesla. So I get to watch in real time how much max regen kW is available, and what power flow (positive or negative) is happening at any moment. I also get to watch my brake temperature.
The bottom line is that I commonly leave my garage with between 40 and 80 kW of regen available, despite just having charged to 100%.
As I go down that hill, I use some power to accelerate, and use some regen to slow for stoplights, At steady speed on the hill I am in mild regen (say 5 to 10 kW, typical). As I go down the hill, my available max Regen drops off substantially, getting down to as low as 12 kW (which is not very much stopping power, by the way), but I have never seen it reported as zero. Quite short periods of acceleration from stoplights give a surprisingly large recovery in Max Regen.
But even mild braking for a stoplights gets me to needing much more braking power than the available max regen. I can instantly see this in rising brake temperatures, and even get a rough feel for "how much". If the brakes went up from 85F to 90F, they barely got a touch. If they go from 85% to 140F, that was a substantial event.
Going back to the original question, it is clear to me that Tesla puts non-zero regen power flow into my battery even when it is reporting 100% SOC, but the rate is severely limited.