The rules in the playtest packet are that the DM gets to decide when a roll is warranted, and that a roll is never warranted when the DC is lower than 5 or higher than 30.But as soon as you assign a TN of 20 to the Sage-background, Proficient-in-Religion cleric, the wording of the rules that say that a roll is warranted when the TN is no less than 5 and no greater than 30 would be allow the player of the Brute Barbarian to say "hey, a roll is warranted, oh, I made a 20". I think this sentence is badly written if they intended to say that a roll is warranted when the DM determines it considering the character who attempt it AND the TN is between 5 and 30. But this is an aside to the problem (which wasn't "who can roll" but "how is getting an auto-success on 20 something that change things").
Let's assume that the character is indeed proficient in Religion. IF you made that character roll, it means you estimate that he can succeed (because we all agree that DMs don't let player roll their dice for the sound of it, but only if they have a chance of succeeding, if only not to have to deal with the player saying "why one earth did you make me roll, I got a 20 and yet I fail?!?"). Then, in the 5e ruleset, you assign a DC. Why are you asking for a roll if you have determined that the DC would be unreachable for this character? And if the DC is reachable, it means that the character would have succeeded anyway on a 20 since it's the best possible score. Honestly, when I know I'll set a DC that can't be reached, for example 25 to lift a cart and the character has 15 STR and is level 1 (so max roll 24), in 5e, I just tell the player "no need to roll, it's impossible". This is the only case where the auto-succeed on 20 would change something: when the DM as said "sure, it's possible, you CAN roll for it" while having set a TN that is out of reach for the character...
Basically, I only see this rule changing anything in the odd case where the DM says "it's possible" and yet assign a TN outside of the character's ability to succeed anyway.
It does not follow that a roll is always warranted if the DC is between those numbers. That's where the "DM gets to decide" part comes in.