If you're getting back pain from an office chair then your arse is likely too far forward when you're sitting and you're putting pressure on your spine due it being at an angle other than 90 degrees from the seat, or your table is too low, lowering your arms, so you're bending forward.

You're suppose to feel your arse pushing against the back of the chair not leaving enough of a hole between the chair and your lower back that you can fit an arm in it, and when your arms are resting on the table (which they should be pretty much all the time if your keyboard and mouse are sufficiently forward) you should feel no pressure either downwards or upwards on your shoulders

I've been coding for over 3 decades, often for massive long hours (to the point that by the age of 17 I had RSI due to how my wrists were resting at the edge of the table and some years later when already doing it professionally went to the doctor with chest pain - which I feared were due to a hearth condition - which turned out to be work posture related) and at some point in my mid 20s I moved to The Netherlands and to a company which had its own Ergonomics Consultant (this was back in the peak of the 90s Tech boom so there was lots of money sloshing around) who would come around when you joined and adjust everything for you (they even had tables with adjustable height) and explain you all about the correct work posture.

Been following that advice and haven't had posture related problems since then whilst always using pretty standard office chairs (always with adjustable height, tough).

I have however seen plenty of people doing the lazy (and stupid) posture of being all the way forward on their chair and quite a lot with arms too low or too high (which is more understandable since most cheap office tables don't have adjustable height).