Hi Osamu,
Thank you for your response.
In the documents that you linked, I can only find information about the requirement that backwards-incompatible ABI changes require changing the SONAME of a library. However, I have not found explicit information stating that security updates and Debian point releases do not allow libraries to change their SONAME (and hence introduce backwards incompatible changes).
Moreover, the scenario I described requires that there be no ABI changes at all (not just backwards incompatible ones): if a Debian point release introduces a new symbol in a library, I may inadvertently link to that symbol, even though my client's embedded machine with an older point release does not have that symbol.
If you could please point me to an exact location in the Debian documentation that says that no ABI changes are allowed between Debian point releases and in security updates, that would be great. If not, I would encourage you to expand documentation to explicitly state so.
Best regards,
Jakob