I didn't think about mine at all either; and it was great so long as we had power. When we lost power on Mon/Tues for 8 hrs it was enough to kill it. Next go round of multi-day sub-freezing temps I'm going to isolate it, kill the gas and power to it, and drain it best I can. Thankfully we have another tankless on the other side of the house so we still have hot water, and also thankfully we can cut off the water flow just to the tankless so I can keep water on inside the house while we deal w/ this. Small blessings.
I've done enough home improvement stuff over my lifetime to be dangerous. I can handle small electrical, carpentry, and other projects - I remodeled one my first houses including paint, reflooring it with hardwoods, moving some walls, carpentry, countertops, moving electrical outlets, etc. But plumbing is one area I haven't gotten into and I'm probably smart enough now to not try, it just seems like the risk of fucking things up is high when you don't know what you don't know. At least in electrical if you fuck up you find out immediately (I found out pretty quick once when wiring up a socket that I turned off the wrong breaker and it was live...oops), but in plumbing if you do it wrong you may have a leak behind a wall for awhile before you figure it out.
But here's a question...in electrical work, if I want to turn off a room or appliance, I can go trip a breaker and work on that section in isolation. Why doesn't plumbing have this concept? Shouldn't there be a manifold with cutoff valves in a central location that let you isolate different legs of water supply to different rooms, like a breaker panel? Why isn't this a standard feature in construction? Explain it to me like I'm 5...