If asdf-ruby supported fuzzy matching for .ruby-version, it would make it even easier for people to migrate from RVM and chruby. We could remove this disclaimer in the README:

If you are migrating from version manager that supported fuzzy matching in .ruby-version like rvm or chruby, note that you might have to change .ruby-version to include full version (e.g. change 2.6 to 2.6.1).

While people could use the asdf-alias plugin, the experience is not as ideal as having the asdf plugin just work.

People can also set .tool-versions or override their local copy of .ruby-version, but if you don't want to check in changes for a pre-existing project, it's another thing to worry about.