Hi guys, not sure I can answer this one straight away.
I know that in th elast couple of years I have still shown potential buyers sipes in som etyres.
I'll have a closer look through a couple of stock rooms this week.
But I think that, if they ARE disappearing it will be because of the inclusion of silica in tread rubber in recent years.
Silica provides a similar sort of result to sipes. But sipes could be overdone to the extent that shoulder blocks flexed too much and allowed feathering, then uneven wear on the shoulders if there was the slightest alignment problem.
Designers walked a fine line between the benefits of sipes and the dangers of uneven wear, then noise.