Considering an iPad Air 3 but slightly put off by this issue.
"
widespread" is a subjective term, but considering that the white spot issue isn't limited to the Air 3 (but includes models with laminated screens larger than 9.7") is an indication that it isn't particularly rare.
This issue does not effect ALL iPads so you'll receive responses that state their iPads aren't experiencing that issue.
I currently own an iPad Mini 5 (after downsizing from larger models over the years). If I were in the market for a large iPad today, the only one I would buy would be the iPad 8. For whatever that is worth.