Seat tube length only really makes sense for sizing for classic flat top-tube road bikes. It definitely doesn't make sense for aggressively sloped top tubes on carbon MTB and road bikes.
Most vendors these days seem to do Small/Medium/Large (etc) sizing, rather than seat tube measurement. It isn't portable across bikes, but neither would seat-tube length be.
I disagree that standover height has much relationship to fit. There is some practical aspect, especially for newer riders. But you can have a very high standover with a high bottom bracket with similar fit to a lower standover with lower bottom bracket.
Most vendors these days seem to do Small/Medium/Large (etc) sizing, rather than seat tube measurement. It isn't portable across bikes, but neither would seat-tube length be.
I disagree that standover height has much relationship to fit. There is some practical aspect, especially for newer riders. But you can have a very high standover with a high bottom bracket with similar fit to a lower standover with lower bottom bracket.