Can we use Aeotec Wallmote or Cooper RF9500 with ST after custom devices/smartapps are gone?

At this point, I don’t know for sure what’s going to work until an edge driver has been released, and that includes Aeotec devices.

There is a very popular Zooz device with a similar form, and Zooz has committed to that working with the new architecture, so that might be an alternative if you want to buy something new.

And Aeotec does have a link to an unofficial edge driver for the Walmote 7

I haven’t seen anything for the Cooper, yet, though.