Wearing clearly immodest clothes in the church and at the Communion rail in front of Our Lord and the entire congregation IS a public mortal sin. I’m speaking of extreme cases, not clothing that is merely questionable or too casual.
As for a priest putting someone’s name on the door denying entry because he went to the Mass of a sedevacantist priest 15 years ago is ridiculous. First of all, that doesn’t make him a notorious public sinner. The priest may know who he is, but the majority of parishioners after 15 years? Hardly! Secondly, if the priest thinks it’s a mortal sin to attend the Mass of a sedevacantist, who is he to commit another mortal sin by detraction? Thirdly, it’s lacking in class, totally tasteless, and immature.