In 5.2, the underline uno command (.uno:Underline) was modified into a split button, but the original non-split button underline uno command was is useful to be used in various locations (e.g. single toolbar mode) where there is limited horizontal space or when the non-split button look is wanted/needed.
So the changed happened here - https://gerrit.libreoffice.org/#/c/24491/
Maxim Monastirsky committed a patch related to this issue.
It has been pushed to "master":
tdf#101672 non-split underline button
It will be available in 5.3.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.
Thanks again Maxim.
Non-split currency - https://cgit.freedesktop.org/libreoffice/core/commit/?id=56205276698dc6633d8110f628339bb01ae1fcc3