Steel rods became more widely available in the 1920's &, as far as I know, continued to be made on up through the 1950's. There were some around before then - even as far back as the late 1800's, but the idea of a steel fly rod never caught on until manufacturers had learned a good way to taper the shafts of the rods. This was accomplished by extruding the steel into a long, continuous tapered shaft. This would also reduce the weight of the rod considerably, because the extruded rod shafts are hollow. Then the blank would be cut to the appropriate lengths for the rod sections, ferrules added, & on to be made into a fly rod.
But a steel fly rod?? I know, you're thinking that these rods must have been heavy, ugly clubs. Well, the vast majority of them were. A full day of casting these rods might very well leave your hands & wrist very tired. So why even make them? Because back then the only other material to make rods from was bamboo & rod manufacturers were looking for a rod they could market as "indestructible". It's true - you probably will never break your steel fly rod in normal use. The added value of these rods is that, if taken care of like any other fly rod, they will be around forever.
Some of the companies that made these rods were True Temper, Heddon, & Union Hardware. There were others, too but these are the companies I think of when I think of steel fly rods. While there's hardly any records left pertaining to steel rods from these companies, I have to think that they sold fairly well. If they didn't they wouldn't have been made for all those years. Still, based on what's available on the used rod market today, I'd put steel rods at slightly rare - you can find them without too much difficulty, but they aren't everywhere.
Believe it or not some of these rods actually felt good, fished well with a good action & were not too heavy. Honestly, there were a few. They weren't all ugly either. I happen to own a Union Hardware steel fly rod that is decked out in the companies best components & it is a nice fly rod. It doesn't weigh a ton & has a nice action. In fact, it feels like a lot of other fiberglass rods that came after it was made. While some of the steel rods were painted to look like bamboo, mine has a nice, subtle gold paint job.
So it is possible to find a decent steel fly rod.
On the other hand.......I also own a non-hollow steel casting rod from the 30's or 40's made by True Temper. This thing feels like a rug beater & could be used to defend one's self in the event of attack. Still, it's nice to have as an example of what used to be made.
Now, I'm not going to tell you that if you've never tried a steel fly rod that you should go out & buy one. No, you're most likely way better off with what you have. Finding a nice one like mine is extremely rare. However, if you see one at a yard sale or flea market & you've got $10 just burning a hole in your pocket, go ahead & buy it. Then you can impress your friends with your esoteric knowledge of not-really-collectible vintage fly fishing tackle!!