Alright so I started playing holy paladin as my main in WOTLK, and I remember when we first got our mastery. (I didn't play MOP, and I only briefly played WOD so I can't attest too much to those expansions.)
It was terrible. We got a % shield on our heal target that lasted (8?) seconds, which ended up wildly ineffective compared with disc priests, and didn't really accentuate our healing in a meaningful way. I don't know how much it improved over time, but it was common to forge away from mastery for holy paladins, it was a fairly dead stat.
With Legion we got our 'battle healer' mastery, making our heals stronger the closer we got to our target. That isn't all that satisfying for a few reasons.
1) Some raids were incredibly movement heavy, or otherwise completely lacked a way for us to synergize our healing with the mechanics. We don't have the movement necessary to really make good use of it reliably in some of these situations. Avatar springs to mind as a fight that just... sort of sucked for mastery with all the running around required.
2) There isn't a good way to gauge how close you are to your target in tandem with the mastery's effectiveness. Without third party tools it is not reliable to say "I am under 8 yards away and getting maximum utility"; you can eyeball it, but it's not reliable.
3) It is entirely possible for our mastery to be ineffective. If the target doesn't need extra healing, then getting extra healing from our mastery isn't useful. We don't really have HOTs where extra healing would likely be dispersed across more sources of damage. It's mostly spot heals.
4) There was a lack of communication for the interaction of our mastery with certain spells, and this had to be figured out entirely by the community.
Now, I don't think the concept for our mastery was a bad idea, but I think it could've been executed better, and here's how.
Back in Wrath of the Lich King, as a holy paladin you could play as a battle healer. In fact it was much more effective to play as a battle healer than at a range. With a combination of high intellect, Seal of Wisdom (restoring mana on hit), and a very high haste rating, you could ensure you had an incredibly deep pool of mana to use.
From my understanding, having a very deep mana pool is what Paladins have always sort of specialized in; this is still partly reflected today by our single target heals being pretty mana efficient. However, these days we pay the price by having our AOE spells as more expensive casts for their output.
I think that in the upcoming expansion we should have our mastery revised, so that instead of increasing healing based on proximity, you instead reduce the cost of your spells based on proximity. Increasing mastery increases the maximum threshold your spell costs can be reduced by, so at maximum range we still get the effective 0% reduction. An alternative approach to this is that when you heal a player, your mastery returns some (a percentage of the cast cost) of your mana back to you, again with maximum range returning no mana.
EDIT just to clarify:
Method 1 = Spell cost reduced by range. If Holy Light costs 400 mana, and your mastery caps at 20% reduction, then at 8 yards your Holy Light will cost 320 mana.
Method 2 = Spell always costs the same amount of mana, but some of that mana is returned to you. If Holy Light costs 400 mana and your mastery caps at returning 20% of mana, then at 8 yards you will spend 400 mana to cast, but be returned 80 mana. This would function differently with spells such as Light of Dawn, as it would be easier to calculate the return of mana rather than reduction. This is also functionally different in that you would always need 400 mana to cast the spell.
The alternative would likely work better with spells such as Light of Dawn, but that may significantly offset the balance of these spells.
I believe this approach to our mastery:
1) Fits in with our long term specialty with healing
2) Is a much clearer benefit to the user, eliminating pure overhealing from our mastery
3) Reduces complexities with mechanics associated with the current system
By having this system, you could more freely utilize your expensive heals (in this case your faster heals) based on your proximity to the target. This playstyle would make you more capable of spamming Flash of Light to deal with any tank health spikes, but still ensure you're reasonably conservative with distant targets. As primary balancing would be around the spell effectiveness compared with other classes, paladins spells should not be more expensive to cast with the expectation of mastery mitigating costs.