Care to share an example of this backrestless chair? Is it like a regular chair just without the backrest, or has some other differences? Does it have armrests for example, and if not - does it bother you?

I went with an overkill approach at first (as I often do :-) and bought some expensive nicely designed "active chair" / stool that was adjustable high enough so that I could lean on it even when using my desk as a standing desk. It was interesting, but not a game changer at all for me. I don't use standing desks now at all.

But what I have now is this:

https://www.amazon.com/dp/B002FL3LY4

Just don't assemble the backrest at first. If sitting up straight, I just lean wrists on my keyboard wristpad and part of forearms on the desk, no armrests needed either.

Edit: I still use my height-adjustable standing desk, but now it's value is that I could adjust it for the perfect height for my sitting-up-straight position (so no chair armrests needed) and it's been fixed at that height for the last 7 years...

Not sure which one the parent was referring to but personalizing I've been using one of these for more than a decade at this point (I'm sitting on it right now) https://www.varierfurniture.com/en/products

The one I have does have a backrest but because of the way it's shaped you don't actually use it to slouch. It's more there to support when you lean back and want to take a break from typing or something like that.