I think that whoever reworked him back at 9.9 did a good job because it allowed the problematic part of his kit to be gutted like it should have been (his w) while giving him buffs in other areas, similar to Rakan's mini rework (removed his flash/W ult combo but got buffed in other areas). Sadly the rework also introduced new problems in hit kit because there were 2 parts that became very op (His Q and passive). I can understand not being able to see flaws like this before a patch hits live because when you make so many changes to a champion it's hard to see what parts are too strong or weak while in testing and it's harder to buff a champion then to nerf one.
Look I get he is a pretty hated champion when strong but people have said it before and I'll say it again. The reason he was so strong back in patch 9.9 was mostly of his Q having too short of a cooldown and because of this:
This basically allowed him to start every game with a rank 2 ult which of course allowed him to be the biggest lane bully ever! Imagine if Elise could could start every game with her ult at level 2, that would obviously be op. Now we are back at where we started with this champ before he got his rework.
One part of his kit being too op and gutting everything else but the actual problematic part of his kit and now his shield duration got reduced and he has less movement speed. So now he is less tanky and he struggles even harder against mobility champions, he can still catch up just fine to champions without mobility. However, the problematic part of his kit allows him to be super frustrating to champions without mobility and useless against champions with mobility. All they need to do to fix this guy is go back to his 9.9 version, increase his Q cooldown like how it is now instead of how it was in that patch and give his passive scaling instead of having it at rank 2 power all game. I know someone in the comments will say he is getting nerfed again because in LCS his W in proplay is still too good at saving people, so here is my suggestion:
Make his W have a unique champion cooldown similar to Rek Sai's W and Yasuo's E or make it where allies take reduced damage while in his belly instead of being immune to damage. Something like: X% of damage Tahm takes also goes to the ally that is in his belly.