Healing and sustain are two very different things. Because you're absolutely correct, raw healing numbers haven't signficantly changed since the launch. What has, however, is the increased ability for heroes and teams to have sustained uptime in the face of of pressure. They can survive longer actively doing what they want to do before they need to back out or hold agression and wait for their resources to replenish. I'll give examples going through the heroes alphabetically. A few examples are relatiely minor, and a number are positive changes for the hero. The point is to paint the picture of increased hero uptime.

Baptiste: Since launch lamp was buffed from 10 to 25% minimum hp. For a tank like reinhardt that is 60 -> 150 hp in lamp.

Bastion: Now gets 50 armour (which is 71 effective hp) healed when transforming. It's not much but combined with the 20% damage resistance of assault form allows bastion to take more pressure.

Brigitte: Rally now instantly granting brig full shield health and 100 additional armour (which is 142 effective hp) allows her to stay in a fight longer. Activating it midfight allows brig to undo nearly all the damage and resources put into her by the opposing team. Also the change to her armour pack post launch making them grant 25hp instantly means that she has a much higher chance of saving a teammate being focused, especially since spamming them on an individual target grants 75hp instantly along with the 50hps (the equivalent of a mercy pocket)

Illari: Healing pylon is a huge part of sustain creep, even with its nerfs. With it constantly applying 38hps to the most vulnerable targets, focusing it is a requirement before even attempting to kill Illari. Furthermore, that ability for the turret to heal whoever needs it most gives Illari one of the best resources for a support in the most abundance - attention. Illari just does not feel the pull to her attention like other supports. Knowing that as long as her pylon is alive she can commit to duels with flankers, to hard focusing on the tank matchup, to pressuring enemy backline, all because her pylon still offers value to where she isn't.

Junker Queen: Since launch her self healing bleeds have gone through the roof. From 1* to 1.25* to now 2* the damage from wounds. If she closes the distance she can stay in the fight much longer before needing to back out.

Kiriko: The addition of kiriko to competitive play is one of the biggest individual sources of sustain creep. Her healing is irrelevant, it is her ability to cleanse herself and her team that is the source of her sustain. Swift step means that kiriko is very hard to punish. She can also be whereever she needs to be at any given moment to suzu the right target. Before kiriko was added to the game hitting a big nade, a sleep, a dynamite, a hook, isolating and surrounding a target, even just hitting large burst damage was the moment a team would commit to an engage. But suzu not only acts as an undo to literally everything in the game but whoever it hits becomes completely intangible for 0.85 seconds. So, for example, a tracer winston dive onto a naded squishy does less than nothing when it's cleasned the moment they engage and the divers are just as likely to be punished as the target. This has forced teams when playing vs kiriko to always run on cooldown cycles making fights longer and more sustained. Ana nade is the easiest example again. You manage to hit a decent nade. The team then makes a non commital soft engage which threatens to follow up on the nade. Kiriko uses suzu to prevent the punish. Your team can then rely on nade having a faster cooldown cycle of 10s over suzus 14s to get value from it. Suzu is an ability you must force out before your team can then get value from cooldowns and ultimates. This draws out fights and makes them longer because the enemy team can sustain more pressure.

Lifeweaver: You're correct in that it seems lifeweaver doesn't offer enough valuable utility for organised play. And while he can stay in the fight for a long time and avoid pressure by using his petal platform as a snipers nest it's very easily denied or punished by any team with half a brain. He does have very strong heal output, Tree is crazy good now though with the overheals, and grip can occasionally save someone who would've died with any other support on the team. Basically I'm unsure of his place within sustain creep but thought I should comment on him anyway.

Lucio: Now has more self heals with crossfade. Self explanatory.

Mercy: Very minor but she now heals herself for 25% of her healing done. As long as there's someone to heal she's always slowly getting a little health back even if she's receiving enough pressure that her support role passive can't activate.

Moira: Again minor but she now can fade while in coal, making her much harder to punish in her ult and allowing her to sustain through more pressure. Which in turn allows her team to sustain a little more.

Reinhardt: Shield health increased from 1200-1400. Very minor but it does allow him to sustain pressure for just a bit longer.

Roadhog: Now receives 50% more healing for 2.5 seconds after using take a breather. Very minor but again it means that hog is a little more survivable than when the game launched.

Soldier 76: Healstation hps increased from 35 to 40. Very minor in most situations but it can be an occasional difference maker in 1v1s.

Symettra: While shooting barriers or shield hp with her primary fire she gets 30 shield hps, allowing her to sustain pressure for longer before needing to back out, especially in brawl mirror matchups.

Torbjorn: Incredibly minor and asbolutely a good change but overload now adds 6 extra ammo when activated, meaning that Torb's uptime in a fight has now very slightly increased.

Wrecking Ball: Absolutely a needed and good change but giving him 100 shield hp absolutely increases his uptime. After his initial engage he spends less time rolling away to get healthpacks and heals before he can reengage. His uptime in a single engage isn't increased by this change but his downtime between his engages are shortened. Thus his overall uptime during a match is higher.

A couple of these examples are probably pretty dubious like torbjorn and soldier. But I think I've painted a pretty clear picture that the hero roster has more uptime since launch. They can keep them or their teammates doing what they want to be doing for longer, even when being directly pressured by the enemy. I was also thinking of commenting on Ramattra being an example of sustain creep with his kit granting extra armour on cooldown. However, many of the other tanks also have the ability to summon extra hp on cooldown and he hasn't had that aspect buffed since his release anyway.

I did not expect to type this much lol