Why is this built into the module system and triggered by a v2? I don't think there is a compelling reason to do so, and there are several compelling reasons not to do so.
I think I prefer the status quo where there is very strong pressure to remain backwards compatible if your module is used by a few people. This leads to far less churn in the ecosystem and more stable modules.
The article makes the assumption that a major version number is used only for breaking changes, but many software packages use such version numbers for major feature releases (as indeed Go2 seems to plan to when/if they release it). I'm not clear why they should be forced to adopt urls ending in /v2 as well.