In order to make a macro that does /equipslot invSlot item you need to either make an actual macro which can't be edited in combat through an addon or use a secure button which is limited in how much it can be modified in combat. Either way it still keeps an addon from making a smart swap decision in combat.
It stops it from happening in the future, keep in mind this is how pretty much everything has worked. People abused movement functions by making auto travel mods so they blocked it, then they did Decursive so they blocked that too. I don't think will see the API return as-is and would imagine they'll keep some form of lock on it, just not as restrictive as it currently it is.
Yes there is, they don't want you using it to make smart choices on swapping equipment like PoisonSwapper did. It's just that instead of being a blanket restriction, it should be in-combat only (or key press while in combat).