Inboxes cut subject lines by width, and phones are where many emails are seen first, so the phone limit is the one to design for. If your offer sits past the cut, the people reading on that phone never see it.
Preview text (the preheader) fills whatever room the subject leaves. Use it to finish the subject’s thought instead of repeating it. On iPhones with Apple Intelligence, Mail may show a short AI summary instead of your preview text, so the subject line has to carry the offer on its own.
Personalised subjects change length with every name. Test with a long first name, not a short one.