There is a variety of edit page related 'notices' usually displayed above the textarea in the traditional EditPage view, which have similar functionality yet diverse logic for adding them to this page as well as highly differing visual representations.
These notices are also not tracked in core and thus not queryable by API (T45683) and mostly lack structured information to decouple presentation from the data (which would be handy for non-HTML edit tools). These notices are roughly:
User level notices
[] no permission to edit
Page level notices from core
[] newarticletext
[] oldid notice
[] longpage-hint
[] Protectedpagetext (and friends)
[] thisisdeleted
[] recreate-moveddeleted-warn
[] namespace notices; (implemented via editnotice-*) See also https://en.wikipedia.org/wiki/Wikipedia:Editnotice#Editnotices
Site specific Page level notices
[] Page notices (implemented via editnotice-*) See also https://en.wikipedia.org/wiki/Wikipedia:Editnotice#Editnotices
[] Group notices (implemented via editnotice-*) See also https://en.wikipedia.org/wiki/Wikipedia:Editnotice#Editnotices
[] Protection notices (implemented via editnotice-*) See also https://en.wikipedia.org/wiki/Wikipedia:Editnotice#Protection_notices
[] Edit intros (implemented via Javascript) See also https://en.wikipedia.org/wiki/Wikipedia:Editnotice#Edit_intros and {T85372}
Challenge is that these already are heavily customised on many wikis.