If useful, i left a comment in the task that is now tagged as 'resolved'. Perhaps it helps.

https://developer.blender.org/T54955#500514

Hi! I'd like to point out that the § symbol is absent from many popular keyboard layouts so, if possible, I think it would be better if some other more universally accessible symbol was used. Maybe "+=" like some softwares use?

In T54987#501541, @Tomas (Rusculleda) wrote:

Hi! I'd like to point out that the § symbol is absent from many popular keyboard layouts

I second that, my (Portuguese) keyboard doesn't have the symbol at all.

I'd vote to either type = or @ or perhaps both if possible. Bonus points for allowing simple expressions like =*2 or =*2/3

For sure, we need to keep that in mind

On several AZERTY keyboards, @ needs to hold Alt to be done, and this is not well handled on some softwares.

Hey, maybe I missed a discussion about that already, but would it be possible to have something like Rusculleda proposed ? It makes it work more like a Pythonic way and makes more sense ? "+=2" is probably less confusing than "++2" right ?

And I have another question, removing the use of ALT to edit multiple objects properties, does it remove the ability to edit just the active object properties ?

Almost always, if you have more things selected, it's because you want to affect those things. If you select three items and hit Delete, those three items get deleted. It's not just the last one you clicked on. This is normal and expected, and why we even have a state of 'selected'. You want your changes to apply to those things.

If it's really needed to affect only one of the selected items, we could flip the Alt key, so Alt-clicking affects only the active object, but I don't even think it's necessary.

Changing values to be three less could by typed as "=-3"

I think Lucas is right that += and -= make the most sense here, since it's the way to do it in python. Even if users are not programmers, =-3 reads like it is literally setting things to -3, while -=3 gives a clue that something else is happening.

Right, I changed this proposal to use = a while ago.

Indeed, I think that's a great change. I was additionally suggesting that the equals comes after the operation sign instead of before so that it fits with convention. '+=' and '=+' have different meanings. But either way I'm super happy with multi-object editing being easier!

About Custom property . and UI panell generate by python,, is there chance to enhance it work so?

Eg,, now I have many custom properties slider as Panell, to animate rig bone , and shape keys. to animate easy, there are many case we hope to change value, with multi selection. It is not so difficult, to make list for all properties of panell, then edit values by python code. or set keys on current frame about each properties group .

but if blender offer more flexible panell UI for custom prop which user can multi select, with modifier keys,
and bpy offer "selected ID list (or dictionary) " for custom properties, maybe add on vendor can enhance function which only work for currently selected properties

How we know which Properties support multi-object editing? I didn't notice any flags indicating so or not

Currently it just attempts to alter all selected if alt was held down and might just silently fail doing that and just alter the active object.

But to invert this behavior and indicate the possibility before the attempt we have to know ahead of time...

@Harley Acheson (harley) Correct. It needs to check in a different way I think. @Julian Eisel (Severin) any thoughts on how this could/should be done?

Every time I look through this it just makes it obvious that someone else (smarter) would have to do it. LOL.