\<char> possibility is there for every character, because one can never be sure what category code a character has.
The escape is certainly necessary for every character of category code 0, 5, 9, 14 or 15, but you need it also for category 1, 2 when you don't want to affect the brace level counters and category 6 if used inside a definition.
The control sequence after
` should have a name consisting of a single character and it need not be defined. In this context the backslash is simply stripped off and the name is used as a character.
Just to mention the case,
`\^^M is perfectly adherent to the syntax, because
^^M stands for a single character after tokenization.
A curiosity: TeX looks for a space also after an alphabetical constant (with expansion). Thus there is a difference between
because in the first case
\foo will only be expanded after
\number has performed its complete action, whereas in the second case
\foo will be expanded prior
\number has finished. Here I'm assuming that
b has its normal category 11.