On one side there is mobile games made for phones & tablets then there is Console & PC games but no longer a focus on making games for handheld devices with buttons.
I disagree with this. basically by accident the indy community absolutely still makes games exactly like that.
rogue-lites are perfect handheld games for example. and there are uncountable rogue-lites... their nature of doing individual runs to unlock new upgrades after each try couldn't be more perfect for mobile gaming.
then there's the onslaught of Vampire Survivor clones. these aren't quite the same as rogue-lites but have a similar concept... minus the random level structures and in an even more segmented progression. but the main thing of doing a "run" and then buying upgrades makes them perfect for handheld gaming.
then there are tons of 2D platformers made by indy studios. also a growing number of really good 3D platformers with short levels that are perfect for handhelds.
handheld specific games were never really a thing outside of a small number of exceptions. handheld games were usually just more compartmentalized versions of stationary console games, that were slightly easier to pick up and play at a whim.
I mean, what was Donkey Kong Land? it was DKC with slightly shorter levels... that's it...
what was Tetris? a port of a game that was originally designed for a computer.
what was Mario Land? smaller Mario... or the first Kirby game? a simple 2D platformer. Link's Awakening? a slightly smaller Link to the Past
and once the GBA was out that thing was basically filled with "console game but slightly adjusted" or just straight up SNES ports.
games like Pokemon that were directly designed around bringing your handheld with you to interact with other handheld owners (trading etc.) were always super rare. the majority of games were just "console game but smaller scope"
and many Indy games fall neatly into that category. their smaller budget almost necessitates that they are simpler, more compact and more gameplay/arcade focused