Keynote: Can't copy slides from new to old version

Keynote v15.1.1 cannot copy/paste into Keynote v14.5, nor vice-versa. No workaround. After installing the v15.1. or later version of Pages, Numbers, or Keynote, one cannot copy/paste between older versions of these applications into the newer versions, or the obverse.

With the v15.1 and later applications installed, Apple expects you to remove the v14.5 versions as they are end-of-life and will not be receiving future updates.

That is pretty std. behavior, newer versions of apps can usually open older versions files, but newer version have new features and the likes added to their files, which would cause older versions of the program to fail to read them.

This would not be special for Keynote it would span across most apps for most platforms.

Are you saying that a Keynote presentation make with Version 15 won't play in the previous version.

What steps have you taken to try to get it to work?

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?