Why have MTB tyres with strong front / back design disappeared?

This is simply not true. Many high range MTB tires are specifically designated for front or back usage, and practically all tire reviews mention that.

Certain manufacturers have encoded the "front/back" purpose of their tires in their names. Maxxis Minion DHF is meant for front, while Maxxis Minion DHR is meant for the rear. Of course, many people use them differently, depending on their riding style and riding conditions. It is not uncommon to see a "front" tire at the rear, for example.

Tire thread designs have changed somewhat over the time (but so have the bicycles). It is unusual to find tires with the "angled" profile (>>>>>>>) as your pictures show at the higher end of the market, where the rear/back distinction is important.

Modern gravity tires are all more blocky (▓▓▓▓), cf. Schwalbe design, but others look quite similar:

They haven't

https://www.schwalbe.com/en/mtb-reader/eddy-current-rear

"FRONT: • Reliable cornering grip paired with high braking traction. • Excellent transmission of steering corrections throughlong grip edges even when less weight is on the front wheel in ascents. The result: Understeer is more controllable.

REAR: • Maximum propulsion and braking traction throughmassive blocks that transmit sufficient power to the ground, even on the steepest climbs. • Hard wearing thanks to large-volume center studs."

https://www.schwalbe.com/en/mtb-reader/racing-ray

"RACING RAY THE FRONT WHEEL SPECIALIST. The completely new, extra aggressive XC profile paired with the versatile Addix Speedgrip compound makes Racing Ray a XC weapon and an ingenious addition to Racing Ralph."

Also

https://www.schwalbetires.com/bike_tires/off-road_tires/fat_albert_front_477

Also given that rolling resistance is more important at the back due to weight distribution, it's common for riders to mix tyres, so you don't really need a 'front' and 'rear' tyre designated as such, you can just choose two different tyres - a harder compound and/or less grippy pattern at the back, and a softer compound with more knobs at the front. Also different tyre widths is common.

See e.g. here

https://www.mbr.co.uk/news/product_news/best-mountain-bike-tyres-323130