It is worth noting that well-intentioned people like Brad or me may already have created go.mod files with older directives. (See e.g. josharian/intern@98cac2a. I picked that version because that was the version in which an optimization was introduced without which the package is useless.)
My 2c is that go 1 or go 1.0 would be the best magic markers for compatibility all the way back.
I don't think a marker for “compatibility all the way back” is a good idea unless it is actually enforced, and I don't think it's worth the labor to retrofit actual enforcement.
Without enforcement, the marker would give a false sense of confidence: once might expect that a package that successfully compiles with a go 1.0 directive would also compile with an actual Go 1 compiler, but that doesn't even remotely hold today.