This is an overview about the minor incompatiblities. For understanding the reasons, why some incompatiblities are existing, the referred pages have to be studied.
The old undo/redo function is incomplete, and the missing recovery of tag associations may even lead to unexpected behavior (see Undo is Erroneous). With new undo/redo implementation tag associations will be recovered, and even embedded windows, and embedded images will be restored. It is not expected that older applications will have problems with new behavior. And if the recovering of tag associations is unwanted, a simple "bug-fix" is possible with the use of the -undo tag option.
It seems to be impossible to provide a complete undo/redo capability which is not affecting older applications at all without a severe loss in user convenience, and the latter should be avoided. The crux is that the intitial undo capability is incomplete, and this incompleteness is in general erroneous (see previous paragraph), and a priori unexpected.
It is not exludable that a problem will be reported, but it is not expected.
The default value of 50ms is different from the original behavior, the original behavior will be restored with zero milli-seconds (see option -responsiveness. But this difference is only leading to an incompatiblity if the application is doing very strange and time critical things, like popups while the mouse is shortly hovering a range of tagged text during a scroll operation. It is not expected that any application will report a problem.
If any application is using names like "@first,last", "@last,first", "@first,100", "@100,last", for marks or images, a clash will occur. It is not expected that this happens in practice.
The commands edit undodepth|redodepth are introduced in version 8.7, and not yet documented. So the removal shouldn't cause problems. The reason for the removal is the introduction of the powerful command edit info, this command prevents the flooding with new sub-commands with the purpose of retrieving informational content.