As I understand it, and again I have no firsthand hands-on experience with them myself, this is just secondhand knowledge I’ve picked up doing my research over the years for when I eventually need to replace my current heater, ideally tankless heaters should be installed with isolation valves, you’d close off those valves, hook a pump up to them, and pump some vinegar through the heater for a while to dissolve any calcium and such that’s built up. It’s probably technically more of a cleaning/descaling cycle than what we traditionally think of as flushing, but it kind of serves the same intended purpose and some people are terrified of change so old terms tend to stick around.

There’s no tank for sediment to collect in, but there is a bunch of tubing coiling around a heat exchanger and all those coils and bends are potential places for stuff to collect and build up in.

It certainly feels less necessary to me since like you said it’s pretty much constantly flushing itself every time you run hot water, and things like your water quality, water pressure, and how often/long you tend to run your hot water for probably come into play somehow. I’d bet that like with flushing a traditional water heater a lot of, maybe most people can probably get away with not doing it and never have any major issues, but if you can remember to do it occasionally it’s cheap insurance it’s cheap insurance to prolong the life of the heater and make sure it’s running as efficiently as possible.

I’m pretty sure I’ve also seen some tankless heaters installed without isolation valves, so it may be something thats not necessary for some models or in all cases, but in those cases if you do find yourself in a position where you need to flush it, that would of course make the operation somewhat more complicated.

So again in general do your research, read the manual, see what the manufacturer recommends, etc.