Is additional filter required for MCU analog domain when using DCDC and linear regulator?
The filter prevents the noise that the digital logic of the MCU itself generates from affecting the sensitive analog sections of the chip &mdash; the ADC and the PLL. The rest of the power supply chain doesn't really have much to do with this.

Yes, without the filter your ADC readings and PLL frequency will be "noisier" than necessary. Whether this is important to you &mdash; and how much filtering is "enough" &mdash; depends on your application.