I feel like I have a unique perspective on understanding this timeline of events.
First I’m aware there is a long history of Anime that can be described as Isekai before this era. This is about the distinctly modern trend of Isekai as we currently know it being one of the most prolific and defining genres of the current state of Anime.
Winter 2013 through Spring 2017 is a time when there is already a visible increase in Anime that can be described as Isekai, as well as non-Isekai Fantasy Anime becoming increasingly more beholden to JRPG Gamer logic.
But from what I recall the English speaking fandom wasn’t using the word “Isekai” all that often yet, all the AniTube videos talking about shows we now call Isekai that I’ve seen did not use the word, it wasn’t in videos on relevant shows from TrixieTheGoldenWitch, Mother's Basement or Gigguk.
This was also an era when there not only were still Fantasy Anime without this Gamer Vibe but they weren't inherently less common either. Spring 2017 has two highly underrated Fantasy shows I would not even remotely classify as even Quasi-Isekai, Grimoire of Zero and WoldEnd: What do you do at the end of the world? Are you busy? Will you save us?. It’s not that those kinds of Anime ever stopped, some like them still get made, but I do think this was the last time we got two in one season.
And almost every proper Isekai from this era is among those that most who’ve seen them consider among the good ones, whether it’s the highly popular instant classics like, Log Horizon, No Game No Life, Overlord, Konosuba, Re:Zero, and Tanya The Evil, or very distinct hidden Gems like Problem Children, Outbreak Company and Grimgar: Ashes and Illusions. The most disliked Isekai from this era is Gate, which was so not for being seen as “generic” but for its politics, and even then Gate was still a very popular show.
Summer 2017 was the beginning of the Isekai era in full force, In Another World With My Smartphone is the first Anime to truly have all of the vibes of a "generic Isekai” which I say as someone who unconditionally loves it. And it was while it and Restaurant to Another World and Knights and Magic were airing that I recall the English Speaking fandom first really started using the word Isekai. And that is when the state of discourse about Isekai entered the form we now know it.
And I find it fascinating how quickly during that season the narrative became that Isekai is an overused premise people are sick of even though earlier the same year no one was complaining about it.
I started writing this thinking Fall 2017 was the last season without an Isekai, nor did it have anything I’d call a Quasi-Isekai. So for that reason you could argue this was still a transitional period and Deathmarch to a Parallel World Rhapsody in Winter 2018 was the start of the full non-stop Isekai bombardment. But the early part of an era can have some stumble in getting off the ground so I was still willing to start this era in Summer 2017 regardless of that.
However I’d forgotten about Ancient Magus Bride, that show is an Isekai but is like the pre 2017 shows in how pretty highly regarded it is, though I still haven’t watched it. It’s also a Shounen Manga adaptation rather than starting as a Light Novel, so it does still feel like part of the modern Isekai discussion purely by technicality. There was also Recovery of an MMO Junkie which is part of the undeniably connected yet distinct trend of Anime about playing an MMORPG.
And then double checking myself saw that there wasn’t an Isekai in Spring 2018, at least not among what I watched, just the somewhat JRPG like Fantasy show Last Period.
Either way Summer 2017 was still the turning point in my opinion.
And I’d say this era continued into the start of the Pandemic.
About 2021 is when the discourse mutated into a state where some were hoping the trend must be finally almost over while others entered resigned acceptance that this being a consistent part of each season is here to stay. It’s also in this era that Isekai’s own Sub Genres started being defined in common use even if some of the roots of them were already there.