Medium employee here to say that this is not true. Whether a story is paywalled or not is not a determining factor when a story is considered for Boost. Never has been.

The factors that matter for Boost are all included here: https://help.medium.com/hc/en-us/articles/360006362473-Medium-s-Quality-Guidelines-How-real-humans-review-stories-for-Boost

And if you want an even deeper dive on how curators assess stories for Boost, you can review the manual our curators are trained with: https://medium-teamspace.notion.site/Medium-Curation-Manual-How-to-Assess-Story-Quality-01277bfcb0d94894b80f4aa1be76ade9