I am using Kubuntu 18.10. I selected the flat acceleration profile in system settings. There still is mouse acceleration while a flat profile should give a 1 on 1 mapping for mouse displacement (how much you move the mouse, assuming an ideal surface under the mouse) vs. mouse pointer displacement. The log also shows it. When I open X.org.0.log I see the text which I will quote here.

6.836] (**) Option "AccelerationScheme" "none"
[ 6.836] (**) SteelSeries SteelSeries Rival 100 Gaming Mouse: (accel) selected scheme none/0
[ 6.836] (**) SteelSeries SteelSeries Rival 100 Gaming Mouse: (accel) acceleration factor: 2.000
[ 6.836] (**) SteelSeries SteelSeries Rival 100 Gaming Mouse: (accel) acceleration threshold: 4

For many users this is an annoying problem. Windows screws this up too but it would be a positive discriminator for Linux if Linux distros would handle this well, or for one particular desktop if it handles this well. As far as I understand (Arch-forums) the problem has more to do with developers of other software which handles this based upon whatever parameters it gets from the desktop than with developers of desktops like KDE Plasma but I also understand that it is possible for the developers of desktops to solve it by using other software for handling the mouse. My impression is that mostly this hasn't been solved yet because it is not a priority and it is not a priority because only a small minority of the users cares about this.

For those who wonder. Why would you want a flat profile? Because it gives a lot more control. With a linear relation between mouse displacement and mouse pointer displacement it is easy to predict how far you have to move the mouse to get the pointer at a specific place on the monitor than with a non-linear profile of which the mouse pointer displacement varies all the time in function of how fast you move the mouse. You easily get overshoots when you make fast movement.
One group of users who insist on a flat profile are gamers who play certain styles of games, most notable shooter games and certain competitive RTS games (Starcraft2, which runs fine on Linux) and MoBa games (like DOTA2 and LOL). Usually their hand-eye coordination is a lot better than that of most computer users because they use the mouse a lot for gaming, they can handle a high mouse sensitivity and they only need a few cm. to move the mouse pointer from one extreme of the monitor to another without any mouse acceleration and with an accuracy of a few pixels or less when they make fast movements. Gaming wasn't relevant on Linux but this has changed the last few years. Now that more and more games have gotten native support for Linux and Valve has made it easy to play non-native games with Wine and some other tools (DXVK) a flat mouse profile could make Linux an attractive option for competitive gamers.

Mouse acceleration is a remnant of a past in which mice had balls, back then the DPI was limited and without mouse acceleration you had to move the mouse a large distance to get the cursor from one side to another. These days good mice (aimed at gamers) typically have a DPI (dots per inch) of 4000 or more, 2000-4000 in the lower segment of gaming mice, which suffices for most gamers. As a user of KDE I would greatly appreciate it if the KDE developers could arrange a truly flat profile for a next version of KDE Plasma. Maybe in the meantime KDE can offer a set of easy to follow instructions (a script or a serie of instructions for the terminal) with which the users can solve it themselves? If anyone who reads this post has a solution then please share it, I would greatly appreciate it.