Is colophon or copyright notice mandatory?

Yes, they are.
The Telemedia Act defines the information colophons should give.

Here is how your colophon should look like:

COLOPHON

Book Title
Author’s Name

© Year Author’s Name/Rightsholder
All rights reserved.

Author: First Name Last Name
Contact Data (Address and/or Email Address)

ISBN: xxx-x-xxxxx-xxx-x
(Please do not use the ISBN of your print edition)
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