Did you remove those settings because they did not work ?

Except rendering features that were relatives to Blender Internal, those features are useful and used in 2.79.

Hair Grid is a huge help when trying to set-up Voxel Grid Cell Size of Hair Dynamics. That is a must-have for people wanting to use Hair Dynamics.
If 2.8 is not able to be display it, that should be considered as a bug, a todo but not an unused feature.
Same remark is valid for guide hairs. It shows initial state of hair particles animated by Hair Dynamics.

Trails are allowing user to easily create a trail of particles following one particle's path. That is really useful. Animations of particles following a path are very common.
In fact, it is a pity that particle instance modifier does not take into account such trail and that they were limited to Halos.
If they were not limited to Halos but able to be used by objects, everybody would use them.

If Line rendering is no more supported at all, why just suppresing its settings and keeping the Line type in Particle Rendering list ?
What is cool about line rendering is the animation of length of particle. And it can be related to particle speed. So, it was used to create sparkles without motion blur cost.
People will use motion blur. That is not a big problem to suppress them.

Please, don't suppress display properties. They are overlays for particles.
Size of particles may be affected by particle textures. And those circles are representing real size used by particles physics.
When using object rendering, object size can be scaled and what you see is not corresponding to particles interactions. So, that is a useful overlay for particles animations.

Velocity of particles are the same kind of useful overlay when working with force fields, collisions. That helps to find correct settings for particles physics.

Number is also used to debug. If particles are not at expected places, it helps user to know if he has to deal with old particles emited at start of simulation or with more recent ones.

I understand that Blender Internal rendering types are no more supported. But most of things that you have suppressed, here, are more about helping user to create easily particles animations than how to render them.

My understanding is that everything that was hidden was doing nothing in 2.80. If this is not the case, it would be good to get a specific example.

Adding back support for some of these would be good, but it will not happen for the 2.80 release and so there is no point having them in the UI.

My understanding is that everything that was hidden was doing nothing in 2.80.

Those settings were not hidden before this commit. They were just not working.

Because nobody took time to make them work. That was not obvious during UI freeze period that no work at all would be done to provide overlays for particles.
I always thought that area would be cleaned.

But here, you are just suppressing what you never gave attention to.
And there is no warranty that the moment to take attention to particles will happen before several years.

Most of those settings are just overlays to add to viewport. They are not rendering types.
Billboard type was removed. Line Type could be removed as well.

But if people want to use Halo Type just with OpenGL render, why not considering trails support ?
If people will have to use Hair Dynamics and particle physics, why making things a lot more more difficult for them by not supporting a Hair Grid Display overlay and a size overlay ?

It is just a question of providing some visual info to user. It is not related to complex physics or EEVEE/Cycles rendering of particles.

I removed the settings from the UI (they still exist internally), because they are not doing anything in Blender 2.80 currently. Nobody says that they are not useful. We just don't have the manpower to make them work for the Blender 2.80 release. It does not make sense to keep them in the UI if they do nothing.

In that case, Line Rendering type should be hidden, too. It is not supported by EEVEE/Cycles or viewport.

That is right, however this could break stuff again. Not sure if it is worth it.

OK. That is not clean.
Most of people try to avoid Hair Dynamics.
There is probably a way with animation nodes to create trails.

But at least, is it possible to obtain from @Jeroen Bakker (jbakker) to make Size display working ?
Any user who will enable collisions between particles or against a surface will need this.