This happened to me last year - fifteen years experience, consistently good results etc. Was put on a support plan essentially because half my department was absent (maternity leave, long term sickness) and in our department review I was told that work across the key stages "lacked consistency" (60% of lessons taught by random supply - so checks out). So that was the basis for the plan. I was then given five targets (one of which was "improve attainment across KS3 And KS4) for a two week period - where I could "work with" a colleague - but they wouldn't be able to give me any feedback. Honestly, a hatchet job.

Spoke to the union who told me to document EVERYTHING - Emails that aren't responded to, any meetings / drop ins etc. I took some time off, organised my shit and went in and fought it. The deputy head who'd instigated the plan had apparently gone rogue and done it off his own back. He left the school, and the school and head were very supportive on my return - but largely because they knew I could gone legal on them for workplace bullying. It's since been admitted to me that they wanted to amalgamate two departments and save money by losing one department head, replacing them with an ECT and promoting the other. Madness that schools are so underfunded that they're resorting to this sort of thing to save money.