The vacuum reservoir of the brake booster in cars with vacuum servo brakes (whether vacuum is generated by the engine or an electric pump is irrelevant) stores enough energy for 3-4 full applications of the brakes.
Vacuum break boosters only make sense for ICE vehicles where you already have an existing air pump (the actual engine) providing free “vacuum”, they don’t make sense in EVs where you need to an extra dedicated motor to produce vacuum, to power a vacuum booster system, to boost the breaks. Much better off just using the extra electric motor to directly boost breaks, without the whole vacuum system as a middle man.
Early EVs use vacuum break boosters, but only because they were the only economical solution, given there was little demand for electric break booster systems. After all a vacuum system is cheaper, if you have a free “vacuum source”. But for last decade or so there’s been enough EVs manufactured that electric break booster systems are now more economical for EVs.
To answer GP question, the an electric break booster system is almost certainly powered off the low voltage (12V) accessory system, not the high voltage system. So a high voltage disconnect won’t prevent the break booster from working, assuming the LV battery is working correctly.
Is an EV like an ICE in that the 12V bus has power while the car is running even if the 12V battery is dead? In an ICE the alternator puts 13.5 volts onto the 12V bus so a dead battery will prevent a car from starting but it will stay running on a dead battery if boosted to start. I imagine an EV does something similar but I don't know.
The 12V battery dying is only an issue if the car is parked and the high voltage battery is disconnected. Then there may not be enough power to 'wake' the car up again.
Mine failed after ~5 years. Replacement was inexpensive ($128) and Tesla service drove to my house to install it.
Like another poster in this thread, my original model 3 battery went ~5 years (typical 12v failure age in a car), and I bought it for $89(!!) at Tesla. Autozone wanted $125 for the same group battery. I did a DIY replacement. For some reason, that one failed after a year and a half. Just bad luck I guess.
Some EV makers, including Tesla, have switched to Li-ion (often LFP) low-voltage batteries. These tend to be better suited to EV duty cycles than lead-acid, and improve reliability and longevity, as well as saving space and weight.
To lecture us on EV brake systems while repeatedly misspelling the word is making me twitch far too early this otherwise fine Wednesday morning.