I write a lot of code for 9front within 9front, all of which is done through the sam editor. While sam does have a powerful general purpose editing language, it doesn't do syntax highlighting or any sort of language aware tooling (LSPs, jump to def, autocomplete, whatever). However I didn't start off with this, I'm not too old (in the second half of my 20s now) so I did take the tour through Java IDE's, tricked out vim configs and vs code as I was learning how to program. However when I moved to working on 9front I actually felt like the lack of these features made it easier for me to focus.
I like to think of code as not that much different than prose, they are both strings of text for communicating information, typically in a fashion of one thing after the other. I think most people would find syntax highlighting for prose to be more annoying than not (outside of perhaps seeing grammar rules for learning). Once I tried reading and writing code without syntax highlighting I found that it encouraged me to actually read and digest code instead of just skimming it. Compare it to reading prose with and without certain subsections highlighted.
Autocomplete strikes me as optimizing the wrong end of the problem. When I'm writing code I generally am spending a lot more time thinking about the problem space or considering possible implementations then I am having my fingers on the keyboard actively typing it out. In general I think the more you're able to think carefully about code in general the smaller it gets, so I find it hard to believe that by making it easier to quickly dump large amounts of text on the screen you're really gaining much. I think there should be a larger focus on reading and understanding code than writing it.
Stuff like code search is quite nice, and even in 9front we do have some scripts and tooling built-in to help us do that. We have programs like 'Bfn' which can search for a function and send it to your text editor, file names with line numbers can also be quickly sent to the editor as well. I think advancements in tooling that helps people move around in code are generally great, the time spent searching for something is generally not something I enjoy. This was perhaps the nicest part of LSPs in my experience. However I do also think that if you make it quite easy to jump around to lots of different files there is less of an incentive to carefully consider how you're laying out your code. How 9front works where there is some tooling to reduce the monotony but not enough to make it easy to traverse a couple million line java project strikes a nice balance for me.