[MertsA]

For instance, Jeeps with ABS. It's not that the manufacturer is actively doing something to prevent you from bleeding the brakes, it's just that to properly bleed the ABS system, you need the ABS system to activate a certain solenoid while you're bleeding them. This necessarily means that something has to talk to the ECU and tell it to bleed the ABS system. The problem is that that something is proprietary tools and diagnostic equipment that the vendor has no real incentive in trying to make accessible to some small shop or the DIY crowd at AutoZone.

Flushing the brakes is fine on every car I've ever heard of. Bleeding air out of the brakes is where it can get more complicated but I'm only talking about cases where air could've feasibly have gotten into the ABS unit. So if you were flushing out some old brake fluid and you managed to drain the reservoir without noticing it, now you have air in the master cylinder and potentially the ABS unit.