Where can I get technical info about stencil thickness and openings for UFQFPN packeges?

[Bob S]

From whomever is doing your board assembly. It depends on what solder paste they use, how much solder their process wants to see on the pads, what other components are on the board (other fine pitch parts may dictate stencil thinkness). You give then the PCB files which show pad sizes and solder mask reliefs, and they figure out the rest. Usually.

[Tesla DeLorean]

Using the 32 and 20 pinned UFQFPN here. Just using regular design rules and understanding of alignment/shrinkage of soldermask from different vendors, they do their own thing on gerbers to suit their process/capabilities.

Manufacturing does their own analysis on land patterns and parts.

Not sure we've engaged ST on these things, but you could try your local rep or FAE to get specific packaging details or physical samples.