Implement the proposal defined in the Termbox and ParserCache Integration ADR.
User-specific html must not be added to the ParserOutput (cache). The "old" PlaceholderEmittingEntityTermsView Termbox prevents that by rendering placeholders in place of the user-specific markup. The new Termbox has to implement some way to ensure that no user-specific markup (e.g. toggle state, language preferences) is added, possibly by rendering the Termbox for logged-in users at a later point in the request life cycle, e.g. on OutputPageBeforeHTMLHookHandler::doOutputPageBeforeHTML.
This has to be solved before deployment.