The safe option if you're unsure about the pronouns is to ask about pronouns or to use the name. Most people consider it ok to use gender neutral pronouns if you don't know the person yet, i prefer to avoid pronouns entirely in such situations and use descriptors like "that person over there" or "the OP of that thread" or whatever fits in that case - it's a bit more stilted, pronouns are a useful tool in language, but sometimes it's just better to avoid them. The reason i'm not a fan of default they / them is partially because gender neutral pronouns are used for deniable misgendering by terfs, partially it's because my first language doesn't have widely accepted gender neutral pronouns and any usage of neopronouns like dey /dem is highly specific and individual, partially it's because even as a nonbinary transfem, it feels kind shit to me personally to be adressed in a gender neutral way. You can't just assume everyone's fine with neutral terms. My gender isn't neutral just because it doesn't neatly fit into the box society has constructed for femininity. Being nonbinary doesn't make you androgynous by default, a sizeable number of trans people aren't nonbinary and a sizeable number of nonbinary people who do use other pronouns than he / him or she / her do not go with they / them, either.

ofc it's always ok to use they / them when you're talking about groups and use it as a plural, or when you're talking about unspecified people instead of certain individuals. But if you're talking about a distinct person and can't ask them their pronouns, pronoun avoidance works better than just using they / them as a hotfix.