Is using the name "Muslim" in its feminine form on mixed forums – knowing that the user's gender is not visible to members and they treat them as male – considered deception and lying? And what is the ruling on entering mixed forums (whether Islamic or scientific), and what are the regulations for that?

It is better to avoid referring to a woman as "Muslim" or adding the feminine marker (tā’ al-ta’nīth) to it, because, although it encompasses both male and female, its use can give the impression that the referent is male. This is an obfuscation that is not necessitated, but if there is a need, then there is no harm. Writing that the gender is female is of no benefit as long as it does not appear to the members.