However, C1 have hardly left any impression nowadays beyond 0x85 Next Line.
Not restricted to C1. Not much of any control character has still an 'impression' nowadays.
Unicode just ignores other C1 control codes.
Sorry to disappoint you, but Unicode does define all of them as part of the C1 Controls and Latin-1 Supplement page. What to do with them concerns Unicode as much as with (almost) any other character ... not at all.
Moreover, EBCDIC control codes were not compatible, so a large market doesn't adopt it either.
Depends on you each ones value for $LARGE_MARKET, doesn't it? While I do agree that the /370 world is all one needs to know, others did have as well some impact - and most modern stuff is rather based on their heritage.
For example the DEC linage as one of the more prominent ones. Here Terminals did use a good number of these codes.
Were they widely used before?
Yes. Right of my memory the VT220 supports codes like
- HTS, IND, RI, NEL and more for cursor handling.
- SS2 and SS3 for single code shifts
- DCS and ST for keyboard programming
And most important of all: CSI to reduce transmission overhead.
In fact, I always feel that many (if not all) of these extensions were made to fit what DEC had in mind for their terminals.
And to what extent?
Well, there might be many more with other terminal variants. Also, ANSI.SYS for MS-DOS does as well support some, so essentially next to all (MS-)DOS machines running2.0 and up:))