It's worth noting that Portal 2 no longer ships libstdc++ per https://github.com/ValveSoftware/portal2/issues/248#issuecomment-318195320 and library conflicts like that should be brought to the attention of the individual game devs.
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I can see the need for Steam shipping with libraries to ensure that the system doesn't have to do much dependency resolution, but there are some things that annoy us users when setting Steam up (especially on a Slackware system)
libstd++ is so out of date, it causes radeonsi_dri.so to not load at all. Steam will not launch the first time until you go into the ubuntu12 lib folders under ~/.local/Steam and delete it - forcing Steam to load the host one. Upon updating the out of date library keeps coming back and you have to repreat said step. This also affects some games (such as Portal 2) that ship with their own version of libstd++
While you ship with libcef, you do not ship with the matching libnss3. This will cause libcef to throw a fatal error and (luckily) just show a black screen when trying to use the store or the community features. This can be fixed by just copying the libnss3 package libraries from e.g. libnss3_3.28.4-0ubuntu0.16.04.3_amd64.deb into the Steam ubuntu libs folder.