That doc clearly says that the mssql_* functions are the aliases. How
can you leave in the alias functions and remove the masters? I don't
even understand how that's possible, much less desirable.

This document is wrong. Feel free to report bug at php. It's true for sybase_ct extension, but not for mssql based function. Anyway I thought that Steve aliased sybase_* to mssql_* when he made this transition in Debian. I wonder why he hasn't do it.

I've counted on Ubuntu packages for years, but if the directive is to
now treat complete feature removal with so little care or oversight,
then I'll have to look elsewhere for my server platforms.

Please don't be hysterical. Attached patch should do the trick. For now you can just download php sources, go to ext/mssql, apply patch, install php5-dev, run phpize && configure && make && make install. Full recompile of php is not neccessary.

BIG WARNING: I didn't test the patch. Use at own risk, I don't even now if it compiles (it should, but who knows).