So a quick question: Was there any desire to suspect/free Walking Wake after Tera got banned and if not what puts it over the edge in a way that's not the case in 'standard' Gen 9 OU?
Is it a case like Dragapult where Z-Moves break it or something?

in cgou full sun, while still ok and seeing some usage, is nowhere near the level of efficiency of yard sun. yard is a prime partner for walking wake as it is a sun setter that, unlike Torkoal and ninetales, is actually good at setting sun to support walking wake and lets it wreck havoc more consistently. Tork and ninetales are the main sun setters in cgou, but they aren’t actually consistent at doing so because they are terrible mons otherwise and there are no other options, which yard is not. yard + wake is basically just a cancer core and the meta basically has no way of dealing with these kinds of structures consistently, especially balance, fat, and bo. So, no, it was never considered to be free or even have a suspect test. No one in the community wants it back either.
Z moves like drag z are also good options on wake as stuff like subsunny still decimates most balance / Bo structures, but even just good old specs in sun in tandem with an actually good and effective setter is enough to break the tier.
In terms of checks, what do you really have to check walking wake with a partner who sets sun really consistently? There’s Primarina, super spdef invested pex, Tapu Fini, the blobs, and super spdef invested ferro that you could consider. But these Pokemon all have their own flaws in checking wake or just as mons in general. Blobs are pretty niche rn and don’t fit on much structures. Primarina and Tapu Fini both do not have reliable recovery and thus cannot check walking wake consistently throughout a game, along with the fact they aren’t easily slottable on teams. Ferro just gets cooked by flamethrower and wake can also just flip turn into yard. Spdef pex is fair, but that is only one Pokémon who would also like to have a lot of physdef for threats like ogerpon and mega Lopunny. Subsunny sets can also easily overwhelm pex overtime and ko it with a z move at pretty high percents.
And no, this is not a yard being broken situation. If walking wake were free, it would be the culprit behind Sun being so powerful and demanding to check defensively. Yard in of itself is not an issue.