This feels like a big dangerous change to me, especially since the current mode of operation has been in place for 20 years.  Should installing a package result in refreshing the configuration for the whole system, causing changes unrelated to the package?

The installation prefix used is important since it might be into a directory already configured via /etc/ld.so.conf or it might be some directory that ldconfig does not know about.

I see that Ubuntu provides special handling for /usr/local via /etc/ld.so.conf.d/libc.conf:

% cat /etc/ld.so.conf
include /etc/ld.so.conf.d/*.conf

% cat /etc/ld.so.conf.d/libc.conf
# libc default configuration
/usr/local/lib

If one installs into a prefix that ldconfig does not already know about, then it seems that additional ldconfig configuration should be required in order for shared libraries installed there to work correctly.