Posting on social media anonymously without getting shadowbanned

In my experience, VPN is usually the culprit. Even if the site doesn’t explicitly ban access, those IPs are not a secret for long. Spam prevention systems catalog them, identify you as a user masking your IP, and raise your spam score The only mitigation would be to avoid proxies that are shared with other users who may get the IP flagged, something like a personal VPS. But then you face a tradeoff: your public IP will be pretty unique, you may be trackable across sessions