Pixels provide the same security features and updates for the budget 'a' series devices as the regular ones. Pixel 8a is one of the recommended devices for GrapheneOS since it still meets all the current era security standards and still has over 4.5 years of updates remaining despite being 3 generations old due to starting with 7 and launching after the initial set of 8th gen devices.
Motorola will be working towards providing the same thing as part of our partnership with them, but we're starting out with the high end flagship devices due to those currently being required for it.