What do you think an sTSS describes?
In cycling, how much aerobic stimulus you put in speaks to the performance gains and physiological adaptations you get out.
In swimming, it doesn’t. That’s the simplest way to put it.
Measure of stimulus in swimming by pace as is done with sTSS is a fundamentally flawed concept - we go faster with better technique, primarily. Technique often driven by drills and the use of swim toys, often meaning you are adapting better whilst going slower.
The worst way to train swimming is to approach swimming like cycling or running, measuring it like those sports encourages you to train like those sports.
In short, I think this is a pretty bad idea 
. I’ve recorded distance and time in the pool but that has the same flaw, I’d recommend looking at the consistency and frequency of your swims, to get a view on how much stimulus you’re getting.