What do you think was the last core 2010s year?

IMO most of 2018 was still core 2010s but, the later months weren't.

Early 2018 was the last time when there were strong elements of core 2010s. Starting at late 2018, the core elements began to decline.

Summer 2018 would be the twilight of the Core 2010s.

With music The EDM of the core 2010s like Future Bass and Tropical House were on their last legs, and this was the last time mumble rap in the hip hop industry, with Emo rap slowly becoming fully solidified thanks to Juice WRLD and XXXTentacion where despite he died, inspired lots of hip hop artists to take cues from his style. There was also a new form of Trap called Bounce, starting with Drake, this has been increasingly popularized by One Hit Wonder rappers in 2019 and 2020. Rap started getting lyrical again followed by Lil Nas X, Dababy, and Drake's and Travis Scotts Sicko Mode. Pop Rock and any form of upbeat pop was still ongoing, though Billie Eilish was creeping into the mainstream. Latin Pop was still in its prime during this time, though this was the last full year it was popular thanks to J Baldvin and Camilla Cabello. Taylor Swift, Selena Gomez, The Weeknd, Shawn Mendes, and other typical pop singers of the 2010s were slowly exiting their golden or silver years, with newcomers such as Dua Lipa and Khalid coming in. Dancehall was becoming less relevant by the end of the year as well.

With gaming, no surprise here. Fortnite and Pubg have been dominating the charts, making a lot of games like GTA 5, CSGO, Rocket League, and Minecraft seem overshadowed, though they continued being relevant but were out of their golden age. You could say 2017 was the absolute Pinnacle of 8th-gen gaming, and 2018 seems to fall off a little from that.

I know my answer is a little messed up but since I'm busy at the moment, I'll probably fix these answers later down the line.

There were some amount left that you would notice, including yuccie-hipster hybrids or those with some kind of hipster fashion as well, in late 2017 and early to mid 2018 but only a few here and there after based on what seen. 

It was definitely dying throughout the first half of 2017. Hipsters were still very prelavant in 2016 but, pretty much gone in 2018. The whole trend definitely peaked in 2013/14.

Hipsters were overall most prevalent between 2006 - 2016 (atleast in big cities where I'm from).

Yeah but what I mean is that in the first of 2018 it was still visible to some extent, and that hipster light, yuccie-hipster hybrids and some fashion like manbuns and were pretty much a much more visible than late 2018-present.

I think peak modern hipster is 2012 to mid 2014. Technically starting in 2004, becoming more mainstream in 2006, more solidified by 08, taking on its 2010s identity by 2009 and slightly new take in 2013. Growing in popularity from late 2009 to 2012 and drying up and adding new elements between late 14 to 2016 and slowly declining but somewhat collectively present between 2016-2018 ( you can even include 14/15 to an extent) but still havIng some traces from late 2018 to the present. You can pretty much still walk into most stores and get a lot of hipster clothing as well. 

Late 2014/mid 2015 is the earliest period where you see hipster slowly slowing down starting to become toned down and more subdued. Then you have yuccie getting big which people confuse with hipster and some alterations. 

2015/2016 is when you officially and easily notice less and less people dressing with the pure hipster look but still it present from what was set up in 14/15.

2016/2017 excessive tattoos, beards and partially shaved hair with whatever remaining hipster looks still pop out. Even the alt right adopts hipster elements. Yuccies still visible. 

2017/2018 : just the last proper gasp of whatever hipster and hipster related was left 

2018 to present : last traces of hipsters, a lot aspects are not technically out of date yet.