In bpo-37266 we strictly disallowed creation of daemon threads in subinterpreters.  However, this is backward-incompatible for existing users of the subinterpreter C-API (such as mod-wsgi).

Rather than reverting that change I suggest that we make it opt-in through the interpreter config.  That would preserve backward-compatibility.  It would also make it so we can disallow daemon threads in subinterpreters created through PEP 554.  We could also deprecate use of daemon threads in *all* subinterpreters, with the goal of dropping support after a while.