Beehaw didn't come up with /instances. That's builtin to all Lemmy instance. It's part of the Lemmy source code. Designed, written, and tested by the Lemmy devs. It's part of their design for transparency.

Instance operators aren't software engineers. I have no doubt some are, and I have no doubt most are not. I also doubt that most are familiar with rust, typescript, postgres in addition to the Lemmy API along with ActivityPub. I'm sure the average admin can "code in" a banner involving some or all of that just like the average person can rebuild an engine, install custom shocks, and weld together a roll cage. But let's say they do "code in" a banner that only displays itself depending on federation status. Now they have a separate patch set that has to be maintained and tested with each update to Lemmy. And published too because Lemmy is licensed under the AGPL.

So yes, this very much is a limitation of the software. The user should be informed, "you can't comment or interact with this other instance." Was this an oversight of the devs? I would say no. As a software engineer there are some things you don't even think of as problems until they pop up as problems. It's part of the engineering process and evolution of a code base.

And my intention from this long winded comment isn't to rip your head off. Even if I come off like that sometimes 🙃. It's me speaking from my knowledge and experience as an engineer.