This should show it well. I use this type of drivers to control simulation settings on my rigged characters, it's really useful to just have 1 slider for full clothing set which runs on the same settings anyway.

In T88932#1173429, @Evan Wilson (EAW) wrote:

it's really useful to just have 1 slider for full clothing set which runs on the same settings anyway.

While the setting won't always update in real time, it will refresh to the driver's
value once the timeline is reset back to the start with shift+left arrow

Ah so your use case isn't updating the vertex mass while the simulation is running, it's using one value to adjust several cloth sims while experimenting to find the value you want.
@Sebastian Parborg (zeddb) @Philipp Oeser (lichtwerk) can you chime in with your thoughts on this?

The ideal thing is to have them update real time for better feedback but right now it works just fine with me having to restart the timeline to see the effects. I'm pretty sure Vertex Mass does refresh in real time, but there are other settings that don't quite work in real time, few months ago i did submit another report where drivers don't refresh the simulation instantly. T80828

In T88932#1173429, @Evan Wilson (EAW) wrote:

it's really useful to just have 1 slider for full clothing set which runs on the same settings anyway.

While the setting won't always update in real time, it will refresh to the driver's
value once the timeline is reset back to the start with shift+left arrow

Ah so your use case isn't updating the vertex mass while the simulation is running, it's using one value to adjust several cloth sims while experimenting to find the value you want.
@Sebastian Parborg (zeddb) @Philipp Oeser (lichtwerk) can you chime in with your thoughts on this?

Hm, this might seem like a valid usecase.
However the downside of having something appear as animatable (where in reality it is not) outweights the pros in my opinion.
If this is just meant to update a collection of objects to the same value, I think there are other ways to achieve this [while possible not as convenient, true]

I would be interested to hear another opinion on this: @Sybren A. Stüvel (sybren): what do you think?

Oh, and btw. : we might dare supporting the real thing (having masses be really animatable while the simulation is running), see D11640: Fix T88188: Allow keyframing vertex mass in cloth sim

I feel that the title "Vertex Mass on cloth simulations no longer animatable" is misleading, as it was never really animatable to begin with. The fact that the animation system could be used to work around some other issue is nice, but not something we (IMO) should be actively supporting/promoting.

D11640: Fix T88188: Allow keyframing vertex mass in cloth sim seems like a simple enough solution. I don't know that much about the code in that area to give concrete feedback on the approach used. If that patch is acceptable, I'd much prefer that over other workarounds.