As someone who has just about everything (all the classic versions starting from GM5 all the way to GMS1 master collection and the GMS2 package) and use GMS1 but starting to seriously consider switching over to GMS2, I actually
still fairly often use GM8. The reason being that because the format is contained within itself it's much, much faster, easier and less clumsy to just quickly start up a new file to test / prototype something. In GMS1 and GMS2 you need to have or make an entire project folder (which you will have to remember to delete again if you don't want clutter - something I can be very bad at) GM8 starts up much, much faster. When I just want to quickly test something, the process from launching the program to writing code takes me 3-5 times longer in GMS1 and GMS2 than in GM8 and below. So TLDR; it's convenient.
GM8 and below also has
some objective advantages over the newer versions, such as not being sandboxed, being able to run code on the go (although this comes with lesser performance), and functions like adding objects at runtime. Depending on what you're doing, these functions can be nearly impossible to live without - just check the guy on here who's making a fake OS. He's using GM8 because of things like this.
Now, that being said, in my personal opinion, GMS1 and GMS2 are of course
vastly superior in just about every other aspect - features and performance especially. I don't know how I ever aspired to do anything incredible without basic tools like shaders for example, the new audio engine is miles better than the old one, physics are really nice to have depending on what you're doing, the networking functionality is
incredible compared to the old one... Oh, and just the workflow; when I do start up GM8 I very quickly get annoyed at little things I take for granted in GMS1, like being able to freely click around from code editor to code editor. Just as an example.
Also, a quick look at the room editor and the sprite editor from GMS2 will absolutely
devastate anything older in the GM lineage.
But like
@Lukan already pointed out, most of this (except editors and workflow related issues) there are DLLs and workarounds for. However, for me, the absolute biggest reasons to use GMS1 and GMS2 as my "daily driver" beyond all the reasons above is continued support (official and unofficial alike), much better performance, an ever expanding array of tools (you won't get new features in GM8 or even GMS1) and of course, multiple platform targets.
But to answer your original question: yes, even with all the love I just sprouted over how much better everything post-GM8 is, I still find GM8 very relevant, and my everyday workflow would be worse without it.