Interesting. I pronounce the "t." Saying it as "offen" sounds lazy to me, like saying "dat" for "that" because the TH sound required too much effort. If I encountered that in curriculum, I would be so confused.
You think pronouncing the T in often sounds proper? Huh. That is a big cultural shift from when I grew up.
Lots of words have a silent T in the middle. Google gave me this list: apostle, bristle, bustle, castle, fasten, glisten, hustle, jostle, listen, moisten, mortgage, often*, nestle, rustle, soften*, thistle, trestle, whistle, wrestle.
According to that website, the * indicates that CAN be pronounced with the T. So apparently, some people do pronounce the T in soften??