Now that the code changes are published, from what I can tell that build plate type selection choice is dependent on the printer configuration preset named not_support_bed_type.
For example from the latest printer definitions, in
system/BBL/machine/Bambu Lab H2C.json:
"not_support_bed_type": "Cool Plate;Smooth PEI Plate / High Temp Plate",
Changing the value to "Cool Plate" or simply "" restores those options (after Studio restart). This also affects which plates are available in a few other UI areas like per-plate type selection, calibration wizard, & printer plate selection when sending a print.
So either editing the existing preset file or creating custom machine config(s) with that property changed should “fix” it. If editing the existing config, be aware it will likely get overwritten during a profile or Studio update. OTOH using custom printer configs may require separate copies for each nozzle size.
EDIT: Changing the value only seems to work in the base printer profile (as shown above). Trying to manually override not_support_bed_type property in custom printer profile files (which are per nozzle size) doesn’t have any effect.
[ Note that this has nothing to do with the printer itself recognizing plates, if “plate detection” is enabled in those options. Just purely for being able to have separate filament settings per different plates, w/out creating filament profiles for each “unofficial” plate. Or eg. running custom g-code based on plate type. ]
-Max