Version: libicu76-76.1-1.1.x86_64 installed 2024-11-26
libicu75-75.1-1.1 worked fine, no error messages.  

On every outgoing mail message, postfix/smtp exudes : 
"warning: ICU library initialization failed: U_FILE_ACCESS_ERROR".  
common/unicode/utypes.h (in what dir? ICU sources?) says it means "The
requested file cannot be found".  These mail messages had no Unicode in headers
or body.  The mail seems to be handled normally.  

From the dist-upgrade yesterday, messages seen in syslog/debug:
    install libicu76-76.1-1.1.x86_64: success
    erase libicu75-75.1-1.1.x86_64: success
    erase libicu75-ledata-75.1-1.1.noarch: success
Making me suspect that the non-found file may be in libicu76-ledata.  The only
file in this package is /usr/share/icu/76.1/icudt76l.dat .  After I installed
the package on all the x86_64 hosts, U_FILE_ACCESS_ERROR was no longer seen. 
It was already installed on aarch64, no messages. 

I suspect there's a dependency issue: libicu76-ledata provides its own name,
and although libicu76 provides libicudata.so.76 it doesn't require
libicu76-ledata (8Mb download, 30.4Mb installed).  And postfix requires
libicuuc.so.76 but none of the data providers.  There are about 12 relevant
requirers on my system, probably more that I don't have installed.  I think
I'll avoid wasting people's time trying to figure out better dependencies for
something I don't really know anything about.