Do I think this archetype is viable? In green maybe, but for the other colors it's rather thin with payoff. It could be a nice alternative to hexproof/auras for green though, to give that color at least some synergy.
I didnt check your lists, but yeah there is more synergy with +1+1 counters than just the phrase.
personally i havent seen anything that pulls me into the archetype. powerlevel is lacking and i dont think its that much of an interesting mechanic either. i might be wrong on the latter. the statgame is kinda boring though imo.
There is a ton of mechanics, which involve (can involve) +1/+1 counters and scryfall doesn't filter you cards where it's only written in the (rules text), like Bloodthirst or Unleash. Not sure if it's possible to search in rules texts aswell.
Yeah persist is also a pseudo payoff. From among the Spikes I think only Spike Colony could be considerable. I also forgot Megamorph, of which at least Ainok Survivalist is decent.
I think the most interesting part of this mechanic is instant speed Proliferate. Stuff like Outlast, Adapt and Graft is all pretty darn slow, but Proliferate can be insane if you have like 3-4 dudes with counters, especially mid combat.
I think this is mostly interesting if you really want something for green, which isn't hexproof/auras, a much more powerful, but also very uninteractive and boring archetype. Then you could have a few random payoff cards from other colors (not like there is much anyway) to pair with green.
unless you want a monogreen mechanic thats goes deeper than "low"ramp manaelf into 3drop, I would recommend building deep into the golgari graveyard theme.
I also wouldnt dismiss hexproof aura that much. edicts and deathtouch can interact with the archetype
On Scryfall you can use fo: to search full oracle text, which includes reminder text.
The main issue I found with trying to build +1/+1 counters was the build up. You need good creatures that start with counters to make the cards that proliferate work. I think with enough scrutiny you could make it work, but I'd be afraid it would require a drafter to gather all the cards. It was better played as tricks in the deck than an archetype.
for the record my cube is 540 in size so supporting the theme and making cuts for it isn't that hard as in a 360 one.
I don't consider it too be a fully fledged archetype but it's more of a theme which, when you draft into one of the pay-off cards early can be a rewarding goal. The payoff cards for me are Ainok bond kon, hagra constrictor, the three green trample enablers and spike colony. Moving around counters as an instant effect shouldn't be underestimated. On that note I should be adding helium grafter and perhaps vigeon hydropon.
@Hoojo - I'm not running any proliferate cards as I find them too conditional. I'm almost exclusively running cards that are solid in their own right.