This may not always be the case at least when it comes to using WP Discourse for SSO.
We’re using WP Discourse + WooCommerce for subscription / membership signups, and I see in our logs we have many Discourse users being auto-deleted by system. We add new WP subscribers to a custom Discourse group using add_user_to_discourse_group in WP Discourse, which as I understand it creates the user, but inactive / pending email verification in Discourse.
Sounds like this can happen with WordPress whenever you’re using a signup flow that’s different from the WP default (for example with Woo or a membership plugin), and therefore still requires email validation on the Discourse end as mentioned here: Auto activated accounts for WP Discourse SSO?
In this case this behavior (auto-purge inactive users) seems okay, as it represents people who signed up on the WP site but never got around to using Discourse. Just thinking through this further…I assume when an inactive account is automatically purged, if that user comes back later they will still be able to activate a Discourse account with the same username? Hope that’s how it works anyway!