This gift was given to players of Pokémon HOME, as a reward for completing the National Dex. Initially, due to issues with the detection code, players had to shift their entire living dex in their boxes in one session, in order to unlock this Mystery Gift in their Pokémon HOME.
This event also marks the first time Original Color Magearna became available to players.
It appears that this gift was initially intended to be released at launch, however they weren't ready to release it; they instead accidentally gave out a Lv.10 Magearna that had 15 ribbons attached to it.
The unlocking of this Mystery Gift temporarily stopped rolling out for public. However if it was already in your Mystery Gift box, one could still claim it.
There were a few reports of a Lv.10 original color Magearna with 15 ribbons presumably because of this.
(image from Japanese site that had the information. It was since removed)
Do note, in the first version of this gift the PID was fixed. It also appears the recipient cannot have a TID/SID combo what would allow one to be shiny.
The Pokémon Home 3.0 update made some changes to the Mystery Gift behaviour. The Rev. 2 of this Gift granted players a Pokémon with random PID, Encryption Constant and HT/WT.