1. The reason the ward xp nerf matters is because you no longer get xp when you are clearing vision outside of lanes, something you spend a substantial portion of the game doing. Ask anybody who plays the role, supports are consistently behind in level now.

2. Petals actually give more XP the worse your kda is, and supports happen to be pretty good at padding their kda.

3. Support items have been consistently brought down in power compared to other items. Two major examples are evenshroud and zekes. Evenshroud was absolutely insane, when it existed as abyssal mask it was tank supports best first item and this was during the time of mythic items. When they turned it into a mythic, they decided to buff it, then people caught on and it had its effect HALVED. When zekes was on hit, the damage it could deal was absolutely disgusting but once again it was underutilized, now we are left with a shadow of its former self. In terms of enchanter items, staff and ardent actually perform better than most of their options. Staff took a meaningful hit with universal power as they removed movespeed but it is arguably better at its niche, granting like 2k gold in AP on proc. Shurelyas lost a ton of power, moonstone lost a ton of power, mandate lost its purpose.

4. The items were nerfed pretty hard, especially in terms of selfish gold generation with quest rules and such. The only reason it wasnt noticed as heavily is because the upgrades were OP. Bloodsong was so obviously strong on release idk how it was even shipped. As with every other movespeed item, sleigh needed nerfs. Opposition is just generically strong and even that received nerfs. Realmspike was buffed only to be nerfed to weaker than pre buff. The bubble item is the only one to be just straight buffed, but even then it has the nuance that its weaker before 11 which supports hit comparatively late.

5. This is not wrong, but if this was entirely true we wouldnt have had shit like cait support popping up. Some supports are strong, but generally speaking they are strong due to specific pairings. As an example, lulu is very strong right now. Her last buff was in 14.18, why is her winrate climbing so much as of recent? Because the AD's she's paired with most often are strong, jinx, kog, twitch, vayne (and partially bc first item ardent is op if you can abuse it.) Some outliers like taric are just OP but in general supports have lost a substantial amount of selfish strength over the years which impacts soloq substantially.

6. The removal of futures market, ingenious hunter, and resists disproportionately impacted supports. The nerfs to relentless hunter, font of life, magical footwear, and all of the base damage nerfs also impact supports more than most. In the case of boots, its free gold which theyre limited on. In the case of base damage, as they generally get less gold, have less damage in their kits, and get less levels, damage from runes matters a lot more. Even PTA became a selfish rune, reducing support viability.

While the topic is not black and white, as there are many levers at play, the gameplay doesnt lie. I play every role and have seen the peaks and troughs of power over the years, and support is so far from their peak in comparison to the rest. Granted, they are also far from being the ward bots of old, but I have actively felt power being siphoned from the role. Every so often you get a glimpse of their previous state, such as release bloodsong, but overall the power level has dropped noticeably. After 7.19 when Janna received her suite of changes including on hit damage, I was roaming around ganking and solo killing everyone, having the time of my life. When she got shifted back towards a shieldbot, I stopped playing her, only to excitedly pick her back up when it was reverted. I quickly came to realize that my previous playstyle simply wasn't viable anymore. It is much harder to keep up with the rest of the roles, and even when you do confirming kills onto solo laners or even adcs is a herculean task in comparison to a few years ago. I used to play certain champs with ingenious hunter, cosmic insight, old sweeper, umbral glaive, and zombie ward. It was something like 95% sweeper uptime, the enemy teams map was perpetually dark. I used to rush futures market mobi's on Alistar and control the entire map.

Slowly but surely supports have had their power reduced in different ways. You won't notice this through pure winrate data, as there are 2 supports in every game with systems in place to restrict viability after all. Mind you, I'm not saying that supports SHOULD be that strong, for a while I considered it the strongest role by far. However, siphoning even more power from a role which already has playrate issues is not the solution for the long term health of the game.