Problem/Motivation
#2228093: Modernize theme initialization removed the ability for themes to implement system-wide alter hooks and instead limits to a certain "whitelist" due to the nature of the caching system in 8.x. This "whitelist" does not contain hook_element_info_alter().
This is a critical hook that should allow themes to alter element information before the callbacks in #process and #pre_render are invoked.
This blocks the port of Bootstrap which uses this hook to replace core (as well as other contrib) and injecting it's own additional #process and #pre_render callbacks. Bootstrap currently has > 50k installs in 7.x.
Proposed resolution
Add this alter hook back.
Remaining tasks
Create patch.
Create tests.
User interface changes
None.
API changes
Allows themes to implement hook_element_info_alter() again.