Thank you for bringing this up again, because yes, you're right, there are still bugs here, but looking more deeply at the callers of can_wear_armour, I suspect some of them are incorrectly passing the desired temp bool, probably because this function is flipping the convention for temp bools used elsewhere in the code.

Also, I'm inclined to take the inventory highlighting as correct behavior(?), because it's being highlighted in grey while the player can't equip it.

Re: Your latest force push, I think we don't want to get rid of the temp check completely, because we have at least one check in invent.cc _has_temp_unwearable_armour that would be broken for bardings if we did.

(It's possible some behavior of these callers may not need to be preserved, but it does seem like it needs to be reviewed.)

Also, I'm inclined to take the inventory highlighting as correct behavior(?), because it's being highlighted in grey while the player can't equip it.

It does look right, but this isn't what happens with other items like boots as only permanently useless items are greyed out in that menu.

Re: Your latest force push, I think we don't want to get rid of the temp check completely, because we have at least one check in invent.cc _has_temp_unwearable_armour that would be broken for bardings if we did.

(It's possible some behavior of these callers may not need to be preserved, but it does seem like it needs to be reviewed.)

We check for temporary things that disable wearing boots later in can_wear_armour so this check wasn't needed. It was also wrong as it would print "You can wear that only in your normal form." for boots using races. We could correct the check if that would be better, something like

if (!you.can_wear_barding(false))
{
    if (verbose)
        mprf(MSGCH_PROMPT, "You can't wear that!");
    return false;
}
else if (!ignore_temporary && !you.can_wear_barding(true)
{
    if (verbose)
        mprf(MSGCH_PROMPT, "You can wear that only in your normal form.");
    return false;
}

The reworked can_equip_item function doesn't have this problem