I don't see why Roku should do that. Most communities have some sort of recycling programs located locally nearby. And older electronics might pose some sort of hazard to the postal handlers assuming Roku even had a remote interest in electronics recycling. Huge unneeded costs to Roku.
Older Roku devices are discarded like everything else. By donating them to thrift stores or simply tossing them in the trash.
Unfortunately, they are worth so little after a few years there's no value in attempting to recycle them for money. I wouldn't just toss them in the trash, however. There are many electronic recycling locations that will take them for free and ensure they don't just end up in a landfill.
But before you get rid of it, make sure you do a hard factory reset to remote all personal information from the player. There isn't much there, but any channel you've logged into would still be accessible by anyone that powered it back up.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.
If you message the refuse company that services your area they can tell you where to bring your discarded electronics. There is almost certainly a processing center near you. If not, it’s worth donating- makers and other tinkerer-types can find uses for parts!