I've noticed a segment on my daily ride is not showing up on Strava anymore. Google led me to a thread at Strava support with many similar complaints but nothing from Strava.

Some have posited that GPS troubles might be to blame; but I've ridden this segment three times in a row and it has stopped appearing as a segment each time.

I saw another opinion that if the rider who created the segment leaves Strava, the segment will go away also.

This is not a heavily-traveled street so I wonder if Strava just eliminated the segment for lack of participation. (I'm a clyde/fred and was fond of the segment because I was in fifth place, a mile with about a 30ft climb).

Does anyone also notice this phenomenon, or know why it happens?

I saw the same thing a while ago.

Easy answer: Just make your own segment, and all the stats will reappear.

Could it have been flagged as dangerous for some stupid reason?

I've ridden segments that were flagged. They still show up, but they just don't have the ranking. Here's one that I know of:
https://app.strava.com/segments/864851

Truthfully, I didn't think it was any better/worse than anything else on this ride. I've been on more hazardous segments that aren't flagged. I think someone got PO'd that he got kicked out of a KOM and flagged it.

I doubt this one would. It's a straight mile with about 30 ft. of gain, on a very wide neighborhood street.

I recently had most of my results on one particular segment suddenly disappear. Only two showed up out of several dozen times I rode the segment. I initiated a "trouble report" with Strava and over the following several weeks, they worked on the issue, going back and forth with me, to finally solve the "problem". Contact them and they are pretty responsive. (Not sure if you have to be a paid member to be able to use this service. I am a paid member.)

Original segment creator can also delete the segments.

This.

Also, I had a segment on my route to work that got flagged as dangerous. There is some very limited visibility at the beginning, but almost no traffic along there. I just made my own segment and made it private so I could see my own progress (its the steepest hill climb on my way to work).

I'm sure you are aware that Strava hides less popular segments - check to the bottom of the list and it should show a link to display hidden segments.

Or try recreating it - Strava will tell you if that segment already exists. I created a segment once that I KNOW tons of people have ridden over, and I'm 1/1 on that thing. Go figure.