I'm thinking what would be the purpose or added value of a separate "public spec" document (which would require a significant maintenance effort) when you already have [1] a healthy Github (or Gitlab) project governance going on, [2] Bitcoin feature proposal process.
[1] https://github.com/bitcoin/bitcoin/projects
[2] https://github.com/bitcoin/bips
When you upgrade your Bitcoin node software, the act of the upgrading is you casting your Yes vote for the changes introduced in that version. The BIPs etc. can help you understand these, but if you want to be 100% diligent, the only fully authoritative source is the state of (the code sources in) the git repo as referenced by the commit hash in the Bitcoin software you are upgrading to.