In Ubuntu 16.04, Super + F brings the dash file search where one can quickly find a file by its name. Very convenient, I use it all the time.
Is there an equivalent in either the GNOME flavour or the MATE flavour of Ubuntu 18.04?
In GNOME in Ubuntu 18.04, you will find files by name by typing part of the name in the search box in the dash. This, without the need to have the GNOME file indexing service, tracker, installed, which is not installed in Ubuntu by default. "Files" must be checked under "Search" under "Settings".
In Ubuntu 17.10, a search in the dash would not bring up files.
MATE standard has no such build-in feature, but a dedicated search panel plugin may be available.
Alternatively, you may want to consider a dedicated quick-launcher. The Albert launcher is very good in what you are seeking, finding files by name to launch them. You could assign it the shortcut Super + S. Other launcher may also do well, but personally, I did not find one that performs like Albert for this task.