You're shifting the burden of tracking from the dev to the build system. Pants ensures deps are congruent, rather than forcing you to do it when deciding what IC to set

it’s not useful to anyone to know what the constraints of the code would be if it didn’t have any deps.

Based on what I found doing migrations, I disagree here. There's value in knowing "what is this code itself compatible with", along with also "okay, now include deps: what is it compatible with?" Note that

py-constraints --summary

will show you both that code's own constraints and the transitive constraints. See the CSV in https://blog.pantsbuild.org/python-3-migrations/