Slack has been nagging me ceaselessly to upgrade the GitHub application, so I decided to go ahead and do it. I went to slack.github.com and clicked the "Add to Slack" button, and it redirected me to a page that said I was over the 10 apps quota for a free plan. So I went to app management and uninstalled the old GitHub app following the relevant steps here, and now the old legacy GitHub integration is no longer listed in the installed apps listing, and the listing has only 9 entries.
However, when I retry installing the new GitHub application by clicking the "Add to Slack" button on the slack.github.com page, I get the same result as previously: a message that says my workspace can't add more apps because I am hitting the 10 apps quota. But I am not hitting that quota! I only have 9 apps in the listing now, and should be able to install 10th! What gives?