If you're using an MFM/RLL drive and are upset about losing that last cylinder or two, then you can simply specify a higher number of cylinders when formatting the drive to make up for it. For example, the ST-225 officially has 615 cylinders, but is physically capable of stepping to at least 663...
The trademark for "MS-DOS" is up for renewal this year. It was last renewed in 2014 for a 10-year period:
https://tsdr.uspto.gov/#caseNumber=73470443&caseSearchType=US_APPLICATION&caseType=DEFAULT&searchType=statusSearch
It requires some kind of special interface board, which it may be useless without. The external DB37 port of the IBM 5150/5160 floppy drive controller does not supply power through the connector. Neither does the external floppy port of the Tandy 1400 series laptops.
The OS/2 Museum has a good article about what went wrong with DOS 4.0/4.01:
https://www.os2museum.com/wp/dos/dos-4-0/
I was also disappointed because I thought Microsoft had released the code for the rare multitasking version of DOS 4.0, but it just turned out to be plain DOS 4.0.
I noticed the comment "8/5/83 ARR IBM MAKES HARDWARE CHANGE. NOW WANTS TO USE HALF HIGHT DRIVES FOR HAL0, AND BACK FIT FOR PC/PC XT"
Was "hal0" an early code name for the PCjr?
I do have an XT clone power supply with a single 12-pin connector, but it takes a lot of force to plug in. So maybe that's one of the reasons why they split it up.
I also don't get the disdain for considering 2000s systems to be vintage. It's not like we're fixed to a 30-year timespan, and thus allowing in PCs from 2005 will automatically cause your 1975 Altair to be forgotten.
And it's not like it would lead to a massive influx of new users here anyway...