Short answer: It doesn't sound like a holdout is coming. Here's the situation:
Mayfield has set a hard deadline — he told reporters he won't negotiate once training camp begins and expects any deal to be done before that point. He's also said the sides are "not anywhere close" on terms (from the Baker Mayfield page).
But there's no holdout threat. ESPN ran a piece titled "Don't Look For A Baker Mayfield Holdout" — the sense is he'll play on his current deal ($33.3M/year) if an extension isn't reached, rather than sit out. The Bucs could also franchise tag him in 2027 at ~$48M, giving both sides a bridge (from the Baker Mayfield page).
It's a prove-it year either way. Mayfield's play declined after shoulder/oblique injuries in 2025 (64.8 PFF grade), and his long-term future in Tampa depends on a bounce-back. He's motivated to show he's healthy and productive — holding out would work against that.
Bottom line: Expect Mayfield to be on the field for camp. A holdout would be a surprise.