Following the docs on Tomcat I had no problem either migrating my war during build or deploying it in the webapps-javaee directory; the migration went just fine.
But I see no option in the manager to deploy a war as legacy; if you try to do that, the app is deployed just fine but it will of course fail when it calls a legacy API.
Thus it seems that the only way is to either migrate the jar before deploy or deploy copying a war file manually (or with a script, but still far from ideal). Is there a way to have a manager deployed war migrated automatically, just as if it was copied in the legacy directory?
Of course the perfect solution would be to migrate the code... or to not use tomcat 10 until you feel like doing what's needed to make it work flawlessly.