How Barbarians will play PvP in 2021?
This is only my opinion, as a Barbarian player, about my class in 2021.

We come from the last skill patch, where PWI changed the effect of golden glyph on Violent Triumph (100% critical hit and 60-80% increased damage on the next hit) and add a few team support tools, top of them Roar (force all the players in a range to target you for 8 seconds) and the new golden Violent Triumph (redirect the damage taken from an ally to you).
Without the boost of Violent Triumph, playing a "glass cannon" build with max Strength and attack level stone has become obsolete and, the increase of strong magic damage dealer with God of Frenzy weapon and the need to use the new support tools, put the word END on this build.

In 2021, both Vitality/Strength builds are good options, but every kind of build, to be competitive, need to focus on two factors:
1) Survivability
2) Team Support
Both, survivability and team support, are connected. Need to be able to tank damage to use the new tools (Raging Slap, Roar, golden Violent Triumph) and be usefull in the team. Then if you want to focus more on survivability, going sage and full vitality, or focus on damage, going strength and demon, are both good options untill you are able to provide your help inside the team.

The two points where to focus for a competitive Barbarian build are:
1) Defense level shards
2) Purify Spell on weapon
Also this two points are connected. Purify Spell become crucial on Barbarian because we are the only class without any kind of teleport/leap/jump, we have skills to tank damage but most the time we get stun locked untill we die. Purify Spell is our "last hope", defense level shards within our high HP and skills to reduce damage, give us an high chance to proc Purify Spell and be able to move for attack, use potions/skills or just run back near the team.

Barbarians are back to Tank/Support role, but is still possible to deal high damage?
Sure! Barbarian damage escalate alot on physical attack more than attack level. The change from attack level shards to defense level shards is not terrible. We have some skills that really hurt (Prehisotric Frenzy, Armageddon, Sunder, Ancestral Rage), we have planty of AoE skills, we have debuffs (Devour, Stomp) and most important we have a paralyze skill. Nothing will be easy as before, but working on debuff and paralyze we can be a threat.
Nothing forbid, if you have the chance, to make two different set: one with full attack shards and one for full defense and switch them as needed.