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There's now an upgrade that says "Opponents cannot have more than Bold 2" or something to that effect. Mechanically, 3 instances of Bold 2 have worked the same as Bold 6. It hasn't been explicitly said that Bold and Tough stack just like Pierce, so does it?

Yes. Bold, Tough, Pierce, and other key words with numerical values do stack.
Source: Wizards of the Coast (2019-10-11)    【Link to Ruling】
Tags: Pierce, Bold, Tough