Bastion received some pretty significant buffs in the last patch, significant enough for him to be used pretty heavily in competitive games -- I've encountered many Bastions hiding behind a Reinhardt shield or an Orisa shield, sometimes even being covered by D.Va's defense matrix, being pocketed by either a Mercy/Ana/Zenyatta -- what options do I really have to deal with this threat? Chip damage from afar doesn't really do much now that he can easily tank more damage, and with a healer it is infuriatingly difficult to chip away at his gargantuan EHP.
There has been a lot of complaining recently that Bastion can survive a lot of Hero ultimates with little difficulty. However, Bastion still has one caveat; he's still a sitting duck. Now, as you mention, he tends to get pocketed hard by a supporting team, and it would seem even pro teams like NRG and Rouge are building compositions based around a Bastion. Fear not though, for some heroes shine in this new Bastion meta:
Sombra may have lower damage than other Assault heroes, but she has some unique kit that helps her take advantage of Bastion. Her hack is invaluable here, EMP even more so to deal with the group protecting the Bastion (Reinhardt, Mercy, etc). She disrupts the set up completely, throwing down Reinhardt's shield, forcing Bastion out of Sentry mode, and stopping any ultimates those characters might have.
Roadhog, despite his recent nerfs, is still a fantastic hero. Hook is one of the only abilities that will pull Bastion out of Sentry mode, so use it wisely. If covered by a Reinhardt, remember this; Reinhardts almost always love to firestrike, and at that point, his shield will be down. Try to time your hooks as best you can, and even if you don't get the Bastion, you may get another critical Hero, like the Rein himself, or perhaps the Mercy.
Sleep darts will also take Bastion out of Sentry mode, and if you didn't read it before, read it again: Bastion is still a sitting duck. Sleep Dart is extremely easy to hit, considering Bastion's stationary nature and humongous hit box.
While still weak and squishy with no mobility options, the utility of Discord orb in this Bastion meta is crucial; it negates Ironclad, leaving Bastion with taking 10% bonus damage (Discord adds 30% bonus damage while Bastion gets 20% damage reduction from Ironclad, which combine additively according to this source). Couple a Zenyatta with any reliable DPS (Pharah, Soldier, McCree) and you have one hell of a fair fight.
Though her recent nerfs have made her quite a bit weaker, the buff to Defense Matrix has not. D.Va can quite effectively cover for her team against a Bastion in Sentry mode.
Launching grenades over things and around crazy angles has always been fun, and Junkrat is seeing a resurgence, at least on console, to deal with Bastions. His ability to stay in some cover, while still pestering an area or denying access to that area, is unparalleled. Rip-Tire is also still a fantastic tool for getting the defense to scatter, and possibly securing a much needed kill or two to push the fight in the attacker's favor.
Ah yes, our old friend Genji. Despite being one of the highest skill-cap heroes, Genji is shining once more. Reflect is fantastic for positioning yourself against a Bastion, allowing you to get in and harass the enemy team, disrupting their setup by forcing the Reinhardt to turn to address you, or causing the Mercy/Zenyatta to make themselves scarce.
It would appear that some of Orisa's abilities will allow her to deal with Bastion personally, as well as providing necessary cover for her team to do the same. Without testing her out, I cannot say for sure how well it works, but as of 3/2/17 (with Orisa's reveal) it would seem to be a solid anchor tank counter.
These are just a few examples, but there are plenty of combinations that still work against Bastion. Considering too that Sentry mode's spread is always at max, it makes hitting some of the smaller DPS characters quite difficult. Remember, Blizzard developed this new Bastion with tank busting in mind, to throw off the triple tank meta. Sentry mode damage against farther away DPS targets is still difficult.
Keep in mind too that while Bastion may be overpowered at this point, there are a few things to consider.
You will not, or at least should not, be attempting to face a Bastion 1v1. Play with your team; things like taking down a Bastion will become easier than ever.
Blizzard is close to releasing another hero, that only they have insight to the abilities at this point. It is still possible that a hard counter to Bastion will be coming.
Some of this is speculation, but some of it is from my current experience with the Season 4 meta.
The strongest tactic I've seen so far is when the team simply dives with dva holding right click and the entire team focusing down bastion.
You can also break his rein's shield by standing out of LoS of the bastion and DPSing the corner of the shield for free. However, if they're smart and communicating, the bastion will simply move for a few seconds and let the rein recharge his shield. Then again, sometimes making the bastion move is all it takes to storm the point.
If he has no rein shield, repeeking with a pharah, widow, or hanzo, or sleep darting or hooking him are all viable.