CanOptIn is whether POST /v1/pricing/enablement/optin would do anything here: the item is in beta and this org is not on its list yet. False for a caller with no validated org, who has no org to enrol.
Effective is whether the caller's org may use the item right now, which is the field to branch on: true for any ga item, for a beta this org holds, and never for an off one.
OptedIn is whether the caller's org is on this item's beta grant list. It can be true on an "off" item — the list survives the kill switch and is simply ignored while it is thrown — so it does not imply Effective.