Planned obsolecense? When we’re talking about an 8-year-old computer?
I think it says a lot of good things about Apple that this computer is “a perfectly capable extra machine, except it can’t upgrade past Catalina”.
Give that machine a new Thunderbolt SSD (to boot from instead of its Fusion drive) and maybe a RAM upgrade (easy to do on a 27" iMac) and it should work great with Catalina and continue to be useful for many years to come. Not what I would consider “obsolete”, planned or otherwise. Even if it can’t run the latest and greatest version of macOS, it can probably run all the apps most people need (an office suite, a modern web browser, media player, etc.)
Is Apple’s $300 trade-in offer reasonable? That has nothing to do with being obsolete and is really just a matter of opinion, but I think you’d be hard pressed to get more for a computer (of any kind) of that age, unless you want to chop it and sell parts on eBay.