@jessamyn
this worked for me (at least for now) on the mac app:

quit slack.app

export SLACK_DEVELOPER_MENU=true open /Applications/Slack.app

cmd-option-i, to open the chrome devtools (or pick it out of the menu)

go to console

at the prompt, change the appropriate js w/

localStorage.setItem("localConfig_v2", localStorage.getItem("localConfig_v2").replace (/\"is_unified_user_client_enabled\":true/g, '\"is_unified_user_client_enabled\": false'))

cmd-r reload slack