When did the trend of powerful slimes come about in Anime?
The trend began when the anime called, “That Time I Got Reincarnated as a Slime” started serialization. This show started in 2018. I don’t know if someone already had an anime that was about powerful slimes before the anime “That Time I Got Reincarnated as a Slime”, but I am sure that they were not as popular as this anime. There are a bunch of anime’s that are about slimes that started after the anime “That Time I Got Reincarnated as a Slime”.
If you want more info, here’s a website that I got all of this info from: https://www.cbr.com/slimes-taking-over-manga-anime/.
I hope this helped