Can they be moved to the compat plugin instead and then remove in 7?

These classes have been broken sind Joomla 1.5 and have been deprecated since Joomla 4.3. It is impossible to use them without mostly re-implementing them entirely in your own class. There is NO code out there, which can use these classes. There is no gain whatsoever in keeping them.

I have tested this item ✅ successfully on 933e78d

I have successfully confirmed that those 2 files are deleted and the change to the file plugins/behaviour/compat6/src/classmap/classmap.php have been made in the Download package.

I have tested this item ✅ successfully on 933e78d

Tested with JBT and current 6.0-dev

• Enable 'Behaviour - Backward Compatibility 6' plugin and set 'Classes Aliases' Yes
• Create override: administrator backend -> System -> Site Templates -> Cassiopeia Details and Files -> Create Overrides -> com_content -> article
  • See 'Override created in /templates/cassiopeia/html/com_content/article'
• Edit templates/cassiopeia/html/com_content/article/default.php file in inserting following lines after last use line 23:

  echo "Overwrite is used";
  new JAdapter(null, __DIR__, 'Test');
  class MyTestAdapter extends JAdapterInstance
  {
  }
  

• Create an article and show the single article, it must containt Overwrite is used and should work
• Installed this PR
  • ✅ Checked libraries/src/Adapter/AdapterInstance.php and libraries/src/Adapter/Adapter.php are deleted
  • ✅ Checked AdapterInstance and Adapter are removed from plugins/behaviour/compat6/src/classmap/classmap.php
  • ✅ Article view fails with Class "JAdapter" not found even if 'Behaviour - Backward Compatibility 6' plugin is enabled and 'Classes Aliases' set Yes

    ---

    _{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/43794.}

Thanks for the work on this @Hackwar, nice to get that group of PRs done. Thanks to testers @muhme & @exlemor for testing.