According to AN4488 for STM32F4 (64-pin in my case), the VDDA pin "must" be connected to two decoupling capacitors. Is this needed for digital-only applications? If not, should the pins be left floating? The ADC seems isolated from the rest of the supply.

[AScha.3]

2 caps is "old style" nonsense. ONE 10uF cer. cap is better in any way. just that time, 20 y ago, such cap was non existent . thats why they tried to do put some 10nF , 100nF and 10 uF elko together, generating a nice group of resonances between thees caps. try it yourself, simulate, or just believe it.

one cap, 10uF cer. X7R , and connect to VDD. thats it.

[Tesla DeLorean]

The STM32 have the PLL and RESET circuitry run off the analogue supplies, you really can't leave them unconnected because you're not using the ADC/DAC