I've bounced around a few places since starting to post my own stuff last year. If you mean 'true spirit' as in welcoming, collaborative, and where feedback actually helps you grow? For new writers, AO3's culture of detailed tags and kudos lets you find your niche without immediate pressure. But the comments can be sparse unless you're in a massive fandom. Wattpad has that chaotic, social energy where everyone's commenting as they read, but the algorithm is brutal and it feels more like a popularity contest sometimes. Tumblr microfic and prompt memes? That's where the raw, immediate 'I just wrote this because I love them' vibe lives, but it's hard to build a longfic there.
My weird take: don't sleep on smaller, fandom-specific Discord servers. You'll get three people yelling about your WIP in real-time, brainstorming with you, and that encouragement is pure rocket fuel when you're new. The big platforms have the audience, but the spirit often feels more alive in those smaller, messier spaces where writing feels like hanging out.