Since writing the above, I found this in the MySQL 5.0 manual:
The minimum and maximum lengths of words to be indexed are defined by the ft_min_word_len and ft_max_word_len system variables. The default minimum value is four characters; the default maximum is version dependent. If you change either value, you must rebuild your FULLTEXT indexes. For example, if you want three-character words to be searchable, you can set the ft_min_word_len variable by putting the following lines in an option file:
Then you must restart the server and rebuild your FULLTEXT indexes.
I then returned to the ACP Search Settings page and, despite being here a dozen times before, this is the first time I noticed that Min and Max Characters Indexed by Search are fields in a section entitled Fulltext Native.
I have now updated my Bug Report, indicating that this is not a Bug. Though it does offer the opportunity to improve the ACP Search Settings page by adding the Min and Max fields to the Fulltext MySQL section, as display-only values of the ft_min_word_len and ft_max_word_len system variables. Possibly with a note explaining that these values can only be changed for all users of the MySQL server, by restarting the database server. Hopefully, this would alert the 99% of phpBB users who run on shared hosting that changing these values isn't something their host is going to do. This is especially important now that most shared hosting companies are installing Resource Monitors that terminate any process as long-running and processor-intensive as rebuilding a Fulltext Native search index for anything but the smallest of forums. That leaves Fulltext MySQL as the only practical alternative for nearly every phpBB user. Short of turning off Searching.