I had downloaded source code of modern Yelp 3.26 (shipped with 18.04 LTS) and grep-ed it for scrollkeeper occurencies:
apt-get source yelp
cd yelp-3.26.0
Grepping does not get results in code, only in text files
$ grep -ir scroll.*keeper . | awk '{print $1}' | sort | uniq
./ChangeLog:
./debian/changelog:
./NEWS:
One of the results from Changelog states:
./ChangeLog: Update slightly to remove scrollkeeper references
./ChangeLog: - Switch to using rarian instead of scrollkeeper
But they do not use even rarian in application code:
$ grep -ir rarian .
./ChangeLog: Port to Rarian 0.8 series
./ChangeLog: - Switch to using rarian instead of scrollkeeper
./ChangeLog: Convert search across to use Rarian
./ChangeLog: Now Rarian is fast, don't split out man and info pages.
./ChangeLog: Spoon is now Rarian. Bump min version to 0.5.0
./NEWS:* Port to Rarian 0.8 series
./NEWS: - Remove scrollkeeper in favour of Rarian
./debian/changelog: - Build depend on librarian-dev (>= 0.7.0).
./debian/changelog: + added build-depends on librarian-dev.
So full-text search is not available in Yelp because of removed ScrollKeeper and Rarian indexers.
Notes:
- some patch exists in old GNOME bugzilla #652057 and modern GitLab. But it is not applied on master branch.
if you are using Ubuntu 16.04 LTS you can install build-dependencies, download and apply this patch:
# software-properties-gtk # enable Source code repositories
sudo apt-get build-dep yelp
sudo apt-get install devscripts
cd ~/Downloads
wget https://gitlab.gnome.org/GNOME/yelp/uploads/b51ee8a2dc22f423f06e0293476328e5/xref-handling-and-indexing-for-help-list.patch
mv xref-handling-and-indexing-for-help-list.patch yelp-3.18.1/debian/patches
cd yelp-3.18.1
echo "xref-handling-and-indexing-for-help-list.patch" >> debian/patches/series
debuild -uc -us
sudo dpkg -i ../*yelp*.deb
After installation you can test patched version of Yelp. Launch it, navigate to All Help, press Ctrl+S and enter for example calculator and press Enter:

But it seems that search works only in titles, not in full text.