1st Gen Flash

Flash used to be an HM, and it was for the first three generations of Pokemon. The HM itself is completely missable in most of the games it is in, as they are usually an optional item in all the games they are in, especially in Generation I. In Generation I, you needed to have caught a certain amount of Pokemon to even obtain flash. The next two generations, the Flash HM was in the hands of NPCs that are easily missable.

As for the move itself, it lit up dark caves. In earlier generations, if you didn't have Flash, areas were completely pitch black, and all you can make out was lights at the end of the cave, and your sprite moving in the dark. In later generations, more specifically, generation III, you could see a little spotlight within a cave that required Flash. However, there have been no dark caves pretty much at all since generation six, and the occurrence has been mostly relegated completely to side areas since generation IV.

The move itself just lowers an opponent's accuracy.

Did you obtain Flash in the older generations? Have you ever navigated the game WITHOUT Flash in areas where Flash is otherwise needed to navigate? I've tried navigating without Flash before. In my most recent playthrough of Leaf Green I didn't even bother to pick the HM up, and hell, I got SO LOST without it in Rock Tunnel. It took me over an hour to find the exit without Flash. But, I could never do that in generation I, nope. I'd definitely get lost even easier. I'm glad Pokemon stopped doing this. While dark caves are a very realistic thing, I've never been a fan of that mechanic in the earlier games. I was really glad when Generation IV rolled around, Flash was demoted to a TM and that the only place Flash was ever needed was a completely misssable side area that was optional to go to.

*I am tagging this as generation I, but I'm mentioning all the first three generations in which Flash was an HM in here as a whole.

I dislike HM moves in general since many of them aren't very useful in battles. I used a disposable HM slave for Flash since I didn't want any Pokemon on my permanent team to know that move, and I wish that Flash wasn't needed in the first place.

trying to navigate dark caves without usage of Flash always sounded like far more trouble than it was worth to me, so in the games where you needed it, I always kept something with Flash on me for those parts

Flash should have been used more, I enjoyed it. It gave the game a stronger feeling of exploration. Like the anime whenever a torch was required, we would use Flash. I liked that.

Rock Tunnel in gb/c gen 1 is not a problem without flash, you can see the walls bc their pallettes have one color
Also, I'm playing it in my gbc, but since my cart is a bootleg one, has some graphical glitches and in rock tunnel it shows as if it were lit up even if i don't used or even acquired the hm