PowerPoint allows, that a presentation starts with number 0. The start value of slide numbering is given by the attribute firstSlideNum of element <p:presentation> in OOXML. Even negative values are allowed in OOXML.
The corresponding attribute in ODF is 20.258 style:first-page-number. Here we have the description text
The value of this attribute can be an integer or continue.
But in the schema there is
So my suggestion is to allow at least the value 0 too. LibreOffice has not yet implemented the attribute style:first-page-number, but there exists an enhancement request about more flexible page numbering https://bugs.documentfoundation.org/show_bug.cgi?id=115611 . I have no strong opinion, whether negative values should be allowed. For interoperability with pptx format of PowerPoint or SoftMaker, only value 0 is needed.
We have made a similar change in https://issues.oasis-open.org/browse/OFFICE-3923 for the attribute style:page-number, which is used in text documents.