If a forum is open to public viewing then divulging your own personal sin on that forum is immodest behavior. I don’t mean asking questions about sin in general, or even asking about a specific sin. What I mean is that saying “I did X. Is X a sin? What kind of sin?” And so on. The more detailed, the more immodest. Sins are personal and private, between you and God, or a confessor. They are not for public consumption. There may even be a call for modesty when in a confessional…details not required.
There countless threads about immodesty in how people dress. But there are rarely any threads about immodesty in revealing too much personal information about ourselves.
A few people are choosing to air their dirty laundry (I could call it indulging their appetite) publicly instead of showing a healthy discretion of what they are revealing. I don’t think the fact that anonymity is involved in most forums matters. Being immodest while being anonymous is still being immodest.
Some quotations from the Catechism about modesty and temperance:
Modesty protects the intimate center of the person. It means refusing to unveil what should remain hidden.
Modesty protects the mystery of persons and their love.
It keeps silence or reserve where there is evident risk of unhealthy curiosity. It is discreet.
Everywhere, however, modesty exists as an intuition of the spiritual dignity proper to man.
Purity of heart requires the modesty which is patience, decency, and discretion. Modesty protects the intimate center of the person.
Purity requires modesty , an integral part of temperance
Temperance is the moral virtue that moderates the attraction of pleasures and provides balance in the use of created goods. It ensures the will’s mastery over instincts and keeps desires within the limits of what is honorable. The temperate person directs the sensitive appetites toward what is good and maintains a healthy discretion: “Do not follow your inclination and strength, walking according to the desires of your heart.” Temperance is often praised in the Old Testament: “Do not follow your base desires, but restrain your appetites.” In the New Testament it is called “moderation” or “sobriety.”