Apostrophe is within ASCII, but breaks sites that a) are prone to SQL injection and/or b) are prone to XSS, generally anything where it can be a control charact

Apostrophe is within ASCII, but breaks sites that a) are prone to SQL injection and/or b) are prone to XSS, generally anything where it can be a control character. "é" is not in ASCII, you're thinking of some "extended ASCII" variant, such as Latin-n (where n goes from 1 to 12). Those are insidious, as your "of course é is in ASCII" becomes "of course ř is in ASCII", or something similar. Finding the original encoding then requires out-of-band signalling, or guessing (heuristics ;)). The technical limitations checked here are in bespoke code, not in the OS.