Earlier quoted context omitted.
Would you mind sharing a bit what makes you say it's really hard ?
No problem. It might not seem obvious when you build small-mid size backends, because in that scenario you might have an access token stored in your database that's checked each time someone makes a request. Token revocation is as easy as deleting that access token from your database. Once you start building something at scale, it's harder to revoke tokens instantly. You still need to validate the token on each reque…
To be a cache, it needs an invalidation scheme already.
Also, no one is asking for "instant" consistency on revoking a token, but at least "eventual consistency".