Borrowed power has taken a step back in the Shadowlands, mainly in power levels, but it is definitely still around.
Instead of a new talent row, the max level power gains come from aligning yourself with one of the four covenants that you help during the Shadowlands leveling campaign.
Upon reaching max level, players will be asked to side with one of these covenants to gain their powers which include a mix of passive abilities and unique class-specific abilities that will change how your class plays.
WoW Shadowlands: How To Change Covenant
If you are unhappy about your covenant and wish to change, all you need to do is revisit 'The Enclave' quarter in Oribos.
Talk to a representative of the covenant you are wishing to join to start the process, and during so you will be asked to confirm multiple times that you wish to change.
Players who join a new covenant must start their renown and research from 0 when joining their new allies.
There is no penalty or quest to switch covenant for the first time, but if you wish to switch back this is where it can get time-consuming.
Two quests must be completed before rejoining a covenant, Prove Your Worth and Rebuild Our Trust.
It will take a minimum of a weekly reset to complete these quests, the time you complete Prove Your Worth does not matter as long as you do it before the reset, and then complete Rebuilding Our Trust after reset.
Both quests are the same and require you to complete various tasks in the zone the covenant you wish to join is located in.
9.1 Chains of Domination
If you previously changed a covenant in 9.0 Blizzard has removed the betrayer status you have with them going into Chains of Domination, meaning you are able to switch back to that covenant without needing to complete the two above quests. Source.
This will only be available the first time, and if you switch again you will need to complete the quests.
If you switch back to a covenant you were previously a part of, you will still keep the upgrades that you had before you left.