WD40 sprayed inside the tubes and swilled around as suggested should be part of yearly maintenance for steel frames especially if they are being used in the wet/mud. Prob less important if the frame is kept I’m s drycellar.
Also, after a wet/muddy ride, remove seatpost and put bike somewhere warm to dry out.
All steel rusts but you can delay it from becoming.a problem for quite a long time.
Lighter frames have thinner/stronger tubing which means they will rust throuh more quickly.
Alloy fatigues/cracks more readily than steel.
Each material has their pros and cons but at the end of the day…
Steel is real!