I haven't really seen many apps that have ceased to exist because of iOS 12. Most of the apps that haven't been updated for at least iOS 11 didn't really work well at all.
If app developers think its worthwhile making it compatible with iOS 12 they would've already.
Considering how many apps there are, i think under 1% of all apps would be affected.
I recently moved to an iPhone Xs. Based on the number of apps not updated to support the larger screen (so over a year now since I'm iPhone X), I'd venture to guess that percentage is currently significantly higher than 1%.
No idea about percentages, but many. When I get a new device I take it as an opportunity to start fresh. No restoring from backup. And when getting things set up I make it a 'rule' to only install apps that have been updated in at least the last year. 6 months is preferred. Many many apps don't meet either of those criteria.
Going to echo it's not an iOS 12 things vs more of a display size thing. Saw same with iPads when they came out, took a couple of years for many developers to adjust their screens for the new device screen: lots of apps, including a number of Google apps, were ignored.
As mentioned, from a basic os standpoint, nothing really big between 11 and 12.
Comes down to developer wanting to put in the effort to adapt the app. And if their old app is not making money for them, might come down to them not thinking worth the effort, even if not that big a move for them.
And add: as mentioned, app might have been abandoned some time ago, so, that variable is added to the equation (guessing old games fall into this catagory).