Yep - one of my sites is dealing with #1 and has been for years. I keep plugging away but it's a bit soul-crushing lol.Very Real
How to tell that you have been sandboxed (This can only be quantified in low competition niche/keywords - medium and high competition is different story)
1: your new posts/pages get indexed but overall your traffic remains same, stuck at some point. this means older posts will get de-ranked or de-indexed
2: your new posts not getting indexed - this is hard form of sandbox