The following INT 21H services are listed in Microsoft literature as not
being supported by DOS 5.0+:

18H (undocumented, original purpose unknown)
1dH (undocumented, original purpose unknown)
1eH (undocumented, original purpose unknown)
20H (undocumented, original purpose unknown)
3700H (undocumented; originally queried command-line switch character)
3701H (undocumented; originally set command-line switch character)
61H (undocumented, original purpose unknown)
63H (undocumented, original purpose unknown)

Notes: In general, the repeated warnings about using undocumented DOS
services have been moot.  On all but the most obscure functions,
Microsoft has maintained upward compatibility since the start.

With DOS 5.0, Microsoft documented many "new" services and
labeled them as "available with DOS 2.0 and all later versions."
Such functions are flagged with  wasUndoc  in TECH Help!

DOS Versions (summary of differences from version to version)

Obsolete DOS Functions