Just a feedback to the non-prefixed ones:

-----Ursprüngliche Nachricht-----
Von: David Brown via Std-Proposals <std-proposals@lists.isocpp.org>

So for non-prefixed attributes, I don't see a situation where ignorable
attributes are ever appropriate - though some can have
implementation-defined behaviour which is "do nothing".

If currently attributes are ignorable, especially non-prefixed ones (i.e. with valid syntax today), they should not suddenly start to be hard errors. Otherwise programs need #if conditionals for each use of an attribute to test for the C++ standard.