I'm expecting a single digit integer input, and have error handling in place already if this is not the case. Are the any potential unforeseen consequences by simply subtracting the input character by '0' to "convert" it into an integer?
I'm not looking for opinions on readability or what's more commonly used (although they wouldn't hurt as an extension to the answer), but simply whether or not it's a reliable form of conversion. If I ask the user to input an integer between 0 and 9, is there any scenario in which there can be input that input = input-'0' should handle, but doesn't?
This is safe and guaranteed by the C language. In the current version, C11, the relevant text is 5.2.1 Character sets, ¶3:
In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.
As for whether it's "bad practice", that's a matter of opinion, but I would say no. It's both idiomatic (commonly used and understood by C programmers) and lacks any alternative that's not confusing and inefficient. For example nobody reading C would want to see this written as a switch statement with 10 cases or by setting up a dummy one-character string to pass to atoi.