I think the Phantump/Pumpkaboo point has merit so I'd like to explain why it doesn't convince me that Porygon has to go - I genuinely do believe that Conversion is what breaks Porygon here, and if it had Forest's Curse instead, it would not be nearly as overwhelming.
Normal Z-Conversion Porygon is comparable to Eviolite Agility Porygon, which was a high C/low B set in ORAS and did not see any buffs in gen 7. They both have the same bulk, but Agility Porygon is able to outspeed several major Scarf users, can attempt to sweep more than once throughout the match, and is slightly easier to set up bulk-wise, whereas Z-Conversion Porygon has more power and (if I remember correctly) an immunity to Taunt and Encore the turn on which it sets up. An extra boost to Special Attack is especially significant against slower teams (though +2 18 SpA Tri Attack still isn't strong enough to OHKO even Tauntfoo after Stealth Rock), but not to an extent that would put Normal Porygon above other high or top tier sweepers.
Electric Z-Conversion Porygon has important advantages over Normal Z-Conversion Porygon that make it much more restricting in teambuilding:
1. Thunderbolt actually provides coverage, and with BoltBeam, Porygon is able to hit the vast majority of the metagame for at least neutral damage, allowing it to run Recover, Magnet Rise, or other defensive moves without giving up important coverage;
2. The Scarf users that check Electric Porygon (Drilbur, Mudbray, Sandile) are much more niche than the Scarf users that check Normal Porygon (Mienfoo, Pawniard, Rufflet, Scraggy) if we exclude the existence of either set; and
3. Thunderbolt gets better rolls than Tri Attack.
Electric Conversion Porygon has enough flexibility and raw power to make it feel unreasonably difficult to deal with compared to, say, Scraggy or Shellder, where Normal Conversion Porygon does not. This isn't to say Normal Conversion Porygon isn't viable, because it can beat most of Electric Conversion Porygon's checks quite handily and is still a decent sweeper on its own. Physical Porygon variants aren't super great but they can beat Whirlwind Munchlax if you can't wait until Porygon is your last Pokemon to set up. There are other various niche Conversion types such as Bug, Ghost, and Psychic that have their own sets of checks. This boosts Z-Conversion's effectiveness even further because if you guess the wrong variant, you're almost guaranteed to lose a Pokemon, as opposed to a one trick Trick-or-Treat pory. (For those of you who played Swirlgar in gen 6 LC, you can think of Electric Porygon as Cotton Guard + Calm Mind Swirlix, Normal Porygon as Belly Drum Swirlix, and the remaining variants as Calm Mind + 3 attack Swirlix. Individual Conversion Porygon variants aren't nearly as ridiculous to deal with, of course, but the idea is still the same.)
So it's not the +1 to all stats, but rather those in conjunction with the ability to change checks and STAB moves on a whim that pushes Conversion Porygon over the edge.
I don't see the issue with treating base Conversion and Z-Conversion as the same entity when being able to boost all your stats (when paired with new item Normalium Z) is just a mechanical buff that Conversion received in the transition from gen 6 to gen 7. And I think that (Z-)Conversion is the broken aspect here because we have very strong evidence that Porygon isn't at all broken without Conversion based on how it was in gen 5 and 6, so Conversion effectively makes every Pokemon that uses it broken.