I've been playing 3rd gen games a bunch lately and sometimes it's amusing to look at the TM distribution there through a balancing lens and try to determine why things are the way that they are.
Regardless of which 3rd gen main game you're playing, the Toxic TM is a true one-off. You've got either the one on Fiery Path (RSE) or the one as the prize for beating Koga (FRLG), and that's it (unless you've got access to specific Japanese e-reader cards for Trainer Hill, anyway). Can't buy it anywhere. Can't farm BP for it. Can't get it through Pickup. The move isn't taught via tutor. The Pokemon who learn the move naturally can be counted on one hand, and it can't be distributed to a wider range of Pokemon through breeding. Yet, the TM itself can be taught to pretty much everything under the sun.
The developer logic here seems pretty straightforward: offer a really powerful and unique attack to the player, grant them the freedom to teach it to whatever they want to, but restrict its distribution to a single party member. It honestly kind of makes sense for only the single-player RPG portion of the game, as it serves as a notable point of customization, utility, and resource management.
It's only when you start factoring in the competitive PvP side of the game that this developer choice starts to fall apart, with dedicated players determined to see that the move be placed on any heavily defensive Pokemon who can make good use of it, regardless of the work required to acquire additional TMs or how thematically unfitting it may be for the specific Pokemon in question to have it.
The previous in-game quest logic for Toxic's distribution also gets a little more murky once you get to infinite-use TMs in later generations for the same reason that applies to any powerful TM move. Can't hand out the really good TMs too early in the quest or else they start to monopolize everyone's movesets and make everything play samey. So Toxic gets the same treatment as stuff like Earthquake and Ice Beam, being gated behind a whole lot more in-game progress or work to acquire. And now Toxic is in a weird place where it's not readily available for most of the quest but can instead be put on virtually anything for competitive use, infinitely, no matter how little sense it makes.
Sometimes it strikes me as odd that it took as long as it did for that move's learnset distribution to be cut down so drastically.