As you already saw on Ability to trigger folder rescan for Composer only eight users voted for it during three years. It is just not so popular, so other features which have many more votes have been chosen for development first.
as someone who builds saas companies and owns one myself, you and i both know that it is extremely unreliable to use a voting systems like this to determine the direction of software. i have used plesk 20 years and not once has the 3 owners ever reached out to me asking about features i struggle with or would like to see done.
That is the approach we use and have found huge success with it. I bet you less than 1% of plesk admins even know about the poll system.
Yes, that works if it has already scanned your website and sees that there are multiple folders with a composer.json file.
If you have scanned and then add a folder with a composer.json file there is no way to have the website rescan for composer.json