I drive a Jeep Wrangler and between its manual transmission and the fact that it's less aerodynamic than a cow, I can and often do slow from highway speeds to ~25 MPH without touching the brakes. It's never been an issue.
I don't think that's the usual expectation. Maintaining absolute braking distance is what railways do, whereas cars commonly operate on relative braking distance plus a safety margin accounting for your reaction time.
Over here, both driving schools and other general road safety education talk about keeping 2 seconds following distance (or half your kph-speed in metres, which corresponds to 1.8 seconds following distance), and traffic engineers commonly give the capacity of a single lane of traffic as 1800 vehicles/hour, which again corresponds to 2 seconds distance (this time as measured vehicle front to vehicle front, instead of tail to front of following vehicle).
When slowing down with regenerative breaking it triggers the lights basin on how fast it’s slowing down.
So if it’s just a tiny bit (slowing from 35 to 30 over a few seconds) it won’t kick on, but from 70 to 35 over a few seconds would.