The comment about "reserved" is puzzling because
OSC 50 has been documented in xterm's control sequences since 1991 for changing the font. Here's a link to the X11R5 version.
The typography in the webpage could be improved. Here's a quick polish to help you see the problem:
5 0 -> Set Font to Pt.
These controls may be disabled
allowFontOps resource. If Pt begins with a "#",
index in the font menu, relative (if the next character is a
plus or minus sign) or absolute. A number is expected but not
required after the sign (the default is the current entry for
relative, zero for absolute indexing).
The same rule (plus or minus sign, optional number) is used
when querying the font. The remainder of Pt is ignored.
A font can be specified after a "#" index expression, by
adding a space and then the font specifier.
If the "TrueType Fonts" menu entry is set (the
resource), then this control sets/queries the
Very likely the problem is in the last paragraph, i.e., you were using TrueType fonts and (not just any text) set the font's facename to a value that did not correspond to a font. Fontconfig has ... no useful error checking to speak of ... and will simply "recover" by using a default font, not bothering to tell an application that something went wrong. If you did that, you cannot reset via the menu the TrueType fontname (the obvious choices on the fonts menu only change sizes). Control sequences still work, however.
OSX Terminal complements this situation by having no useful documentation. But some of its features are copied from various places (or may be copied to other places). The profile-change feature appeared in KDE Konsole in 2008 as part of the changes for bug report #155801 (curiously citing a rather old copy of xterm's control sequences document which someone made into a webpage). Actually the
50 was in the code from 2007 (hardcoded). But the change in 2008 gave it a name and commented that it conflicted with xterm's