when you catch all the Unown and that cave appears when you press A on a certain part of the wall another cave appears and deep in that cave you can find Deoxys is this true? (my friend told me this.)
No. Unless you cheat somehow. You can only get Deoxys through Pal Park
Source: Experience and also Serebii. (I looked at locations)
No, it's not true, Deoxys is unobtainable in Platinum without hacking, of course.