Per the title, what are some of the oldest books on algebraic curves out there with unsolved exercises? Maybe there are some hidden gems from before the 20th century out there.

$\begingroup$ I have the impression that exercises started to appear on research level books not too long ago, say the 60s or even the 70s... $\endgroup$ – IMeasy Aug 5 at 20:51

$\begingroup$ @IMeasy there are exercises in all Bourbaki books, the first volumes of which appeared in the 1940's. $\endgroup$ – YCor Aug 9 at 9:59

3$\begingroup$ Same question by OP on calculus, real analysis,linear algebra, complex analysis, nonanalytic geometry, number theory. $\endgroup$ – YCor Aug 9 at 10:02

2$\begingroup$ Very confused as to the point of these questions. Is the OP writing an article on the subject or something? $\endgroup$ – Sam Hopkins Aug 12 at 17:22

2$\begingroup$ @SamHopkins I'm not. For my edification and those of others. The assumed hostility has to do with the unfortunate presentist antiantiquarian attitude of younger blogosphere mathematicians active on MO. Most wellknown senior mathematicians I have talked to in real life about these questions have been quite receptive. $\endgroup$ – user126532 Aug 12 at 19:05
Zeuthen’s Lehrbuch der abzählenden Methoden der Geometrie (1914) has Übungsaufgaben at the end of most sections (§§35, 48, 54, 59, 92, 97, 115, 141, 157, 166, 178, 185, 205).
Magnus’ Sammlung von Aufgaben und Lehrsätzen aus der analytischen Geometrie (1833, 1837) has chapters on Linien höherer Grade (Vol. 3, pp. 241–292) and Flächen höherer Grade (Vol. 4, pp. 377–425).

1$\begingroup$ @user126532 You might generally like this search, which produces e.g. Minin (1911) incl. Aufgaben der algebraischen Geometrie. $\endgroup$ – Francois Ziegler Aug 13 at 1:05
You might find the works of George Salmon (18191903) interesting.
Amongst others, he wrote:
Here is his MacTutor bio.