HI,
I'm only a volunteer, so I can't give you an official answer.
In the documentation there's a section "Material UI Premium" and under Reference a section "Premium",
there are a few objects (usually in green, like CodeEdit) or methods (like Method),
some Samples are Premium. The ability to handle translations in your app is built-in but only documented for Premium Users.
Sometimes Premium features become available to non-Premium users
"External storage access (SAF) is now available to non-premium users."
"Added _sdk_ file detection to enable SDK menu for non premium users."
I've been using DS for a few years and the only significant removal I recall was due to DS getting banned from the Play Store
"Removed support for Admob (Google requirement)."
Occasionally something becomes obsolete or is replaced (this even happens in Premium)
"Please note that CreateListInitialIcon, CreateListMaterialIcon and CreateListWithAvatar are now deprecated.
But still you can achieve these designs by passing appropriate comma-separated options to the options param of CreateListModern method"
There are free plugins (I haven't seen any go from free to fee),
Some plugins require a fee, and AFAIK are quite separate from DS.
In the past you could buy the Builder separately, but now all of DS Premium is one fee (monthly or lifetime).
On Google Play Store you can buy DroidScript UI Kit which as far as I can tell is almost identical to DS Material UI Premium
DS is a "not for profit" organization. They do not seem to be just trying to make money.
They genuinely try to help users, investigate and fix problems, adjust for new versions of Android, add new features, etc
Regards, ah