Yes, it could technically be extended, but the transition would be a massive undertaking, so in practice the encodings do limit the number of codepoints. UTF-1

Yes, it could technically be extended, but the transition would be a massive undertaking, so in practice the encodings do limit the number of codepoints. UTF-16, which creates the limitation, is very widely used and required by major programming language standards like ECMAScript. A lot of software still can't cope with codepoints outside the BMP, and they were established with UTF-16 in 1996.