I searched around for the answer to this question and everywhere it says to hold down the Ctrl key while clicking the blocks you want to copy. I'm on windows and that does not work at all. So has something changed and Ctrl key is no longer the magic key?
As far as I remember, this has never been possible.
What advice is frequently mentioned is to use Ctrl+C.
To copy a block, first select the block. You can copy the block by either pressing the copy shortcut for your platform (Ctrl+C for Windows/Linux, ⌘C for macOS).
See Editing Functions in App Inventor)%20the%20block. 
Where exactly did you find this?
On the test server there are sone more features available to test...
Taifun
On a Mac, holding Ctrl and clicking will also bring up the context menu, which has the Duplicate option. I don't know if it will also work on Windows.
Remembering that you have the option of copying the Blocks individually to Backpack and then Paste them ALL at once »
Lito
@>-->---
Windows 7 Prof, Firefox, Tests: Press "Shift" and/or "Shift+Ctrl" (no difference) and click on the blocks:
Conclusion:
If I need several attempts to successfully select blocks, it is ultimately faster to drag the blocks individually into the backpack.
Windows11, Chrome, it doesn't work. Ok, now I see that you are showing functionality from the test server.
If you need to copy multiple blocks, you need a procedure or a refactor.
Win7, Chrome works the same way as Firefox.
I've reproduced the issue and made a note to have someone look at it.
Is multiple selection an internal implementation or a plugin mentioned on the blockly pages?
It's a plugin that came out of a GSOC project that we managed.