I'm the souce.
I can confirm and, GUARANTEE you it is NOT shiny locked.
I was soft resetting for a TIMID nature Latias since i didn't have a pokemon with sychronize in my team.
15-20 soft resets later a SHINY Docile Latias showed up.
I clearly gave up on the Timid one and kept the shiny one.
Since a shiny one randomly showed up for me while i wasn't trying to get one i can CONFIRM THEY ARE NOT SHINY LOCKED and can be soft rested for a shiny.
I suggest taking a pokemon with sychronize and the nature you want before soft resetting for it.
I suggest timid, modest or calm (defense, raises her already high sp defense) for Latias
Hope this helps