1 You can no longer switch to an older compiler version

This was done with improvement number CDS-74626

• Release Note:
  [[GENERAL]]
  * The choice of the compiler version is limited to only the newest version or the version already used in the project.
     This applies to the project compiler settings and the project environment dialog.
  ...

• If you want to create a new project for an old controller, you should use the newest compiler version anyway.
• If for some reason you need exactly an older compiler version for a new project, you must install that old version and create a project there.

2 Datatype STRING can now be either UTF8 or the old format, ASCII

Check out the documentation https://content.helpme-codesys.com/en/CODESYS%20Development%20System/_cds_utf8_encoding.html

You can enable it with this setting (below screenshot), and all STRINGs will be treated as UTF8 strings.

If it is deselected, you can still have a UTF8 STRING literal, by typing UTF8#'aäoöuü'.
In this case, the string is stored as [61 C3 A4 6F C3 B6 75 C3 BC 00]. If you have the setting unticked and type instead 'aäoöuü', you get [61 E4 6F F6 75 FC 00]

You can also decorate a STRING with this attribute to force UTF8 monitoring.  {attribute 'monitoring_encoding' := 'UTF8'}

3 You can now symbolically access the IO channels

Check out the OnlineHelp for more information on Symbolic Access to I/O Channels.

This is a new feature.
In case of any problems, a workaround is to just use the old access method, which is still possible.

4 Generic constants for Functionblocks

Check out the OnlineHelp for more information on Variable: VAR_GENERIC CONSTANT

These are intended to replace the deprecated Library parameter list.

5 "Offline" Online help and "Online" Online help have changed

help.codesys.com has been replaced with www.helpme-codesys.com, and has become a completely new platform.  Keyword: Faster.

The Offline .chm files are now HTML files, which duplicates the www.helpme-codesys.com website for offline use.