Don’t know if someone has alrady pointed this out, but while using one of my favourite brushes (flatten) i noticed this little thing, then tried to evaluate it even with clay brush, here is the result of my tests
If this is in reference to the new flatten brush in 3.5, please see the first post in the 3.5 sticky at the top of the page for how to make the flatten brush behave as it did in previous versions.
No, it’s not only in reference to the new flatten brush…the brush mod modifier now doens’t work at all…a workaround can be playing with imbed value, but seems that it gives a slightly different response
Yes. The brush mod for the clay brush does not work in 3.5 
As mentioned by Bingo_Jackson (thanks:))
The answer is in this thread
Read the info about flatten brush and find the following…The functionality of the “Mod” slider in 3.1 is now controlled by the Imbed value (which is now a property of all brushes).
so all brush mod = imbed value now? Therefore I don’t need to bother with brush mod anymore right?
Not for the clay or flatten brush.
The imbed value is now common to all brushes which means that all brushes can now use a control which previously was available to only few brushes.
The Mod is still needed. Some brushes, such as the standard brush, uses the mod value as pinch control.