Every few years the major linux disros have a clean up of a problematic or dated sections of the linux universe.
My opinion that you should only add the following package if you are willing to have problems and help to find solutions.
usrmerge is a part of a consolidation that RedHat and Debian (not sure about the BSD's) are working on.
Myself I will start to add the usrmerge incrementally to Virtual Sparky/Debian machines using Sparky5/Buster/Testing and also a SDHC install of Sparky with I3 and connman. As it states in its description - there is no going back once all the symlinks are set up. If you only have one computer or if stability is important on a system, do not add.
More info at
https://wiki.debian.org/UsrMerge
https://lists.debian.org/debian-devel/2018/11/msg00374.html
https://salsa.debian.org/md/usrmerge/raw/master/debian/README.Debian
My first usage will be in a pure debian virtual install, only way to get bugs reported easily.
Edit
Started using
Smooth and no errors on bare metal I3 wm and sparky5 connman instead of nm
Sparky 5.5 dev with lxqt connman xfwm4 sddm I got hit with
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=914409
So I would advise others to avoid trying. until usrmerge 20 arrives from Sid in a couple of days.
usrmerge 20 has arrived. Bug # 914409 is squashed. No problems noted for me.
More base info about the merge usr changes in the linux environment is here.
https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/
I am not sure if new installs of Buster will transition to a default merged usr system in the future.