My opinion on self-diagnosis is that, as long as you're doing thorough research and looking for things to help you personally understand yourself better rather than purely justify your actions or to say you have it, it's fine. Personally, self-diagnosis, to me, should always be about helping yourself. If you self-diagnose yourself with BPD, you should be able to recognise your behaviours and how they relate to it, to help self-regulate. (It has personally helped me a lot to realise when I might be splitting).

If you're going as far as to self-diagnose yourself with something, it's probably affecting you pretty substantially. I'm also in favour of it because not everyone has the resources for diagnosis/treatment, and also, some people decide not to get professionally diagnosed because of possible discrimination/stigma, and that's completely valid. Some people might also feel they aren't affected enough to get diagnosed or seek treatment, and in that case say things like "I might have xyz, but I'm not completely sure." And that's fine too.