RES was around long before new Reddit and it does what it does based on what classic Reddit has in it, and what it gets itself. I don’t know much more detail than that, except that the devs basically lost interest in doing more than maintenance of it after the redesign came out.

should Fediverse platform/app devs spend time trying to accomodate specific shitty platforms?

That’s one way to look at it. I would pose it as

should Fediverse platform/app devs spend time providing the best user experience to the users who are on their platforms?