If you were feeling really brave, you could write upgrade code in mod/lesson/db/upgrade.php to convert the old "lesson dependancy" settings to the equivalent new equivalent activity completion and conditional availability settings.
Then you could drop the old code completely.
Of course, you would need to worry about sites that use "lesson dependancy", but which have either activity completion or conditional availability turned off at Site administration -> Advanced features. However, you could even handle that using a custom environment check (like https://github.com/moodle/moodle/blob/master/admin/environment.xml#L1131 )
This is all theoretical, to point out that it could be done. I am not saying that this is necessarily a worthwhile use of your time.