Some Mystery Gifts are still available through the DNS Exploit, a process that takes advantage of the games' DNS settings to access old events. Any event accessed through this method is legitimate and will pass hack checks. Not every old event is available; for a complete list, and a guide on setting up the exploit, please reference the links below.
How to obtain all the Gen 4/5 mystery gifts? (Pokemon Database) How to unlock Gen 4 and 5 Pokemon Mystery Events in 2018 (Shacknews)
Nope, but see here for more info.
