Coating weapons in substances to make them more deadly has been done since we had weapons, it's called poison. Fire is just a bit more exciting.
Poisoned weapons are extremely rare and have been primarily limited to very specific applications. Applied to a sword or similar? Basically unheard of, if you hit someone with a sword enough to transfer the poison to their bloodstream, they're already badly wounded and the poison isn't going to add much.
If the physics of a world allow flaming swords just by dipping? Flaming swords would be everywhere. It would be a common tactic.
It's the other way around, it was in the game because it's the sort of getting-creative-with-their-environment thing that players want to do. BG3 didn't invent it.
And don't try to deflect by getting caught up in a specific example. How does it profit you as a DM to shut down any attempt to get creative in play with either "well, that just straight-up wouldn't work" or "okay, but you realise I'll be having the enemy do that to you every encounter from now on"? Who is that fun for?
The rules and limitations are there for a reason. I gave examples of why I think it's a bad idea to allow anything and what kinds of things I do allow. If that means I'm not the DM for you, so be it. If you allow that kind of "creativity" you probably aren't the DM for me either.