Technically we don't know that. In the beta they were 20 because shaders probably didn't want to be seen or weren't ready. Since we can get them Pre-Launch I bet they will be waiting to be equipped at one of the levels 1- 8 on lauch day.
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Every single shader seen to date has had a lvl 20 requirement flag, including the Blacksmith shader that people can obtain through a Pre-Order cross promotion with COD:AW & Gamestop. You are correct that we don't know 100% yet, however, all data points at shaders requiring lvl20.