Skype for Linux comes packaged as Electron application. The code describing client behavior is in file app.asar. This file can be edited after emerging to hide the "React" button, which can be done by calling an ebuild hook
- Create
/etc/portage/bashrcwith content from below - Create
/etc/portage/hooks/net-im/skypeforlinux/post-installwith content from below emerge skypeforlinux
The hook logs its actions and saves a backup. In case skype does not start after this - replace app.asar with the backed up version.
This also hides "Expression picker" to the left of chat pane