I suspect the entry-level iPad will get side Touch ID if the iPad mini is. It makes sense to move the whole product line to full-screen devices.
Differentiators for iPad 2021 models:
iPad Pro - M chip/dual camera/Face ID
iPad Air - latest A series chip/dual camera/side Touch ID
iPad - A12 or A13 chip/single lens camera/side Touch ID
iPad mini - A12 or A13/single lens camera/side Touch ID