I don't know what the effect is of using a 4-byte language pack on a not migrated database. I think no problem as long as no 4 byte utf-8 characters are written to it. But that is hard to predict if and when that will happen. There are also loads of other easy ways users can enter 4 byte characters in the database, e.g. making a forum post from a mobile phone and adding an emoticon.
So, honestly, I don't think it is an issue. The feature of being able to use 4byte utf-8 characters went in on 13 March 2017. There are reminders on the upgrade page that converting the database is necessary. That is over 5 years now. No one should run a web app for 5 years without upgrading it or should ignore the upgrade recommendations. Newer installations should follow the installation guide and have 4-byte support.
So, honestly, I don't think it is an issue. The feature of being able to use 4byte utf-8 characters went in on 13 March 2017. There are reminders on the upgrade page that converting the database is necessary. That is over 5 years now. No one should run a web app for 5 years without upgrading it or should ignore the upgrade recommendations. Newer installations should follow the installation guide and have 4-byte support.