That is indeed why they limited it, but that was a mistake. I want to call UTF-16 a mistake all on its own, but since it predated UTF-8, I can't entirely do so. But limiting the Unicode range to only what's allowed in UTF-16 was shortsighted. They should, instead, have allowed UTF-8 to continue to address 31 bits, and if the standard grew past 21 bits, then UTF-16 would be deprecated. (Going into depth would take an essay, and at this point nobody cares about hearing it, so I'll refrain).
Interestingly, in theory UTF-8 could be extended to 36 bits: the FLAC format uses an encoding similar to UTF-8 but extended to allow up to 36 bits (which takes seven bytes) to encode frame numbers: https://www.ietf.org/rfc/rfc9639.html#section-9.1.5
This means that frame numbers in a FLAC file can go up to 2^36-1, so a FLAC file can have up to 68,719,476,735 frames. If it was recorded at a 48kHz sample rate, there will be 48,000 frames per second, meaning a FLAC file at 48kHz sample rate can (in theory) be 14.3 million seconds long, or 165.7 days long.
So if Unicode ever needs to encode 68.7 billion characters, well, extended seven-byte UTF-8 will be ready and waiting. :-D
